<?xml version="1.0"?>
<edgarSubmission>

    <schemaVersion>X0708</schemaVersion>

    <submissionType>D/A</submissionType>

    <testOrLive>LIVE</testOrLive>

    <primaryIssuer>
        <cik>0002000269</cik>
        <entityName>Fortress Lending Fund IV (B) L.P.</entityName>
        <issuerAddress>
            <street1>1345 AVENUE OF THE AMERICAS</street1>
            <street2>46TH FLOOR</street2>
            <city>NEW YORK</city>
            <stateOrCountry>NY</stateOrCountry>
            <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
            <zipCode>10105</zipCode>
        </issuerAddress>
        <issuerPhoneNumber>212-798-6100</issuerPhoneNumber>
        <jurisdictionOfInc>CAYMAN ISLANDS</jurisdictionOfInc>
        <issuerPreviousNameList>
            <value>None</value>
        </issuerPreviousNameList>
        <edgarPreviousNameList>
            <value>None</value>
        </edgarPreviousNameList>
        <entityType>Limited Partnership</entityType>
        <yearOfInc>
            <withinFiveYears>true</withinFiveYears>
            <value>2023</value>
        </yearOfInc>
    </primaryIssuer>

    <relatedPersonsList>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Garth</firstName>
                <lastName>Ebanks</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>Maples Fiduciary Services (Cayman) Ltd.</street1>
                <street2>P.O. Box 1093, Boundary Hall, Cricket Sq</street2>
                <city>Grand Cayman</city>
                <stateOrCountry>E9</stateOrCountry>
                <stateOrCountryDescription>CAYMAN ISLANDS</stateOrCountryDescription>
                <zipCode>KY1-1102</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Director of the Issuer's General Partner</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Kimberly</firstName>
                <lastName>Melen</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>Maples Fiduciary Services (Cayman) Ltd.</street1>
                <street2>P.O. Box 1093, Boundary Hall, Cricket Sq</street2>
                <city>Grand Cayman</city>
                <stateOrCountry>E9</stateOrCountry>
                <stateOrCountryDescription>CAYMAN ISLANDS</stateOrCountryDescription>
                <zipCode>KY1-1102</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Director of the Issuer's General Partner</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Omar</firstName>
                <lastName>Wright</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>Maples Fiduciary Services (Cayman) Ltd.</street1>
                <street2>P.O. Box 1093, Boundary Hall, Cricket Sq</street2>
                <city>Grand Cayman</city>
                <stateOrCountry>E9</stateOrCountry>
                <stateOrCountryDescription>CAYMAN ISLANDS</stateOrCountryDescription>
                <zipCode>KY1-1102</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Director of the Issuer's General Partner</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>N/A</firstName>
                <lastName>FLF Fund IV GP Limited</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>Maples Fiduciary Services (Cayman) Ltd.</street1>
                <street2>P.O. Box 1093, Boundary Hall, Cricket Sq</street2>
                <city>Grand Cayman</city>
                <stateOrCountry>E9</stateOrCountry>
                <stateOrCountryDescription>CAYMAN ISLANDS</stateOrCountryDescription>
                <zipCode>KY1-1102</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Promoter</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>General Partner of the Issuer</relationshipClarification>
        </relatedPersonInfo>
    </relatedPersonsList>

    <offeringData>
        <industryGroup>
            <industryGroupType>Pooled Investment Fund</industryGroupType>
            <investmentFundInfo>
                <investmentFundType>Other Investment Fund</investmentFundType>
                <is40Act>false</is40Act>
            </investmentFundInfo>
        </industryGroup>
        <issuerSize>
            <aggregateNetAssetValueRange>Decline to Disclose</aggregateNetAssetValueRange>
        </issuerSize>
        <federalExemptionsExclusions>
            <item>06b</item>
            <item>3C</item>
            <item>3C.7</item>
        </federalExemptionsExclusions>
        <typeOfFiling>
            <newOrAmendment>
                <isAmendment>true</isAmendment>
                <previousAccessionNumber>0002000321-23-000004</previousAccessionNumber>
            </newOrAmendment>
            <dateOfFirstSale>
                <value>2023-11-02</value>
            </dateOfFirstSale>
        </typeOfFiling>
        <durationOfOffering>
            <moreThanOneYear>false</moreThanOneYear>
        </durationOfOffering>
        <typesOfSecuritiesOffered>
            <isEquityType>true</isEquityType>
            <isPooledInvestmentFundType>true</isPooledInvestmentFundType>
            <isOtherType>true</isOtherType>
            <descriptionOfOtherType>Limited Partnership Interests</descriptionOfOtherType>
        </typesOfSecuritiesOffered>
        <businessCombinationTransaction>
            <isBusinessCombinationTransaction>false</isBusinessCombinationTransaction>
            <clarificationOfResponse></clarificationOfResponse>
        </businessCombinationTransaction>
        <minimumInvestmentAccepted>1</minimumInvestmentAccepted>
        <salesCompensationList>
            <recipient>
                <recipientName>Fortress Capital Formation LLC (the &quot;LLC&quot;)</recipientName>
                <recipientCRDNumber>152385</recipientCRDNumber>
                <associatedBDName>None</associatedBDName>
                <associatedBDCRDNumber>None</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>1345 Avenue of the Americas</street1>
                    <street2>45th Floor</street2>
                    <city>New York</city>
                    <stateOrCountry>NY</stateOrCountry>
                    <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                    <zipCode>10105</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <value>All States</value>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
            <recipient>
                <recipientName>Morgan Stanley Smith Barney LLC</recipientName>
                <recipientCRDNumber>149777</recipientCRDNumber>
                <associatedBDName>None</associatedBDName>
                <associatedBDCRDNumber>None</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>2000 Westchester Avenue</street1>
                    <city>Purchase</city>
                    <stateOrCountry>NY</stateOrCountry>
                    <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                    <zipCode>10577-2530</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <value>All States</value>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
        </salesCompensationList>
        <offeringSalesAmounts>
            <totalOfferingAmount>Indefinite</totalOfferingAmount>
            <totalAmountSold>358800000</totalAmountSold>
            <totalRemaining>Indefinite</totalRemaining>
            <clarificationOfResponse></clarificationOfResponse>
        </offeringSalesAmounts>
        <investors>
            <hasNonAccreditedInvestors>false</hasNonAccreditedInvestors>
            <totalNumberAlreadyInvested>18</totalNumberAlreadyInvested>
        </investors>
        <salesCommissionsFindersFees>
            <salesCommissions>
                <dollarAmount>0</dollarAmount>
            </salesCommissions>
            <findersFees>
                <dollarAmount>0</dollarAmount>
            </findersFees>
            <clarificationOfResponse>The entities in Item 12 act as placement agents for the Issuer, but neither they nor their registered reps are receiving any sales commission, finders' fees, or other remuneration from the Issuer for acting in such capacity in this offering.</clarificationOfResponse>
        </salesCommissionsFindersFees>
        <useOfProceeds>
            <grossProceedsUsed>
                <dollarAmount>0</dollarAmount>
            </grossProceedsUsed>
            <clarificationOfResponse></clarificationOfResponse>
        </useOfProceeds>
        <signatureBlock>
            <authorizedRepresentative>false</authorizedRepresentative>
            <signature>
                <issuerName>Fortress Lending Fund IV (B) L.P.</issuerName>
                <signatureName>/s/ Garth Ebanks</signatureName>
                <nameOfSigner>Garth Ebanks</nameOfSigner>
                <signatureTitle>Director of the Issuer's General Partner</signatureTitle>
                <signatureDate>2024-11-07</signatureDate>
            </signature>
        </signatureBlock>
    </offeringData>
</edgarSubmission>
