<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>00000014 - Disclosure - Convertible Promissory Note</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>From2013-01-01to2013-06-30</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001489367</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="6 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>sqnf_ConvertiblePromissoryNoteAbstract</ElementName><ElementPrefix>sqnf_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>ConvertiblePromissoryNoteAbstract</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>sqnf_ConvertiblePromissoryNoteReceivableTextBlock</ElementName><ElementPrefix>sqnf_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>verboseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="From2013-01-01to2013-06-30" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div&gt;&lt;table cellpadding="0" cellspacing="0" id="list" style="width: 100%; font: 10pt times new roman; font-size: 10pt; font-family: times new roman"&gt;&lt;tr style="vertical-align: top"&gt;&lt;td style="text-align: left; width: 18pt"&gt;&lt;div style="text-align: left"&gt;&lt;font style="display: inline; font: bold 10pt Times New Roman"&gt;8.&amp;#160;&amp;#160;&lt;/font&gt;&lt;/div&gt;
&lt;/td&gt;
&lt;td&gt;
&lt;div style="text-indent: 0pt; margin-left: 0pt; margin-right: 0pt; text-align: justify"&gt;&lt;font style="display: inline; font: bold 10pt Times New Roman"&gt;Convertible Promissory Note&lt;/font&gt;&lt;/div&gt;
&lt;/td&gt;
&lt;/tr&gt;&lt;/table&gt;
&lt;/div&gt;

&lt;div style="text-indent: 0pt; display: block"&gt;&lt;br /&gt;
&lt;/div&gt;

&lt;div style="text-indent: 0pt; display: block; margin-left: 18pt; margin-right: 0pt; text-align: justify"&gt;&lt;font style="display: inline; font: 10pt Times New Roman"&gt;On February 27, 2013, the Partnership entered into a Subscription and Securities Purchase Agreement to purchase a portion of a $3,500,000 Convertible Promissory Note (&amp;#8220;Promissory Note&amp;#8221;). On February 28, 2013, the Partnership purchased a Promissory Note with a principal amount of $1,500,000. The Promissory Note bears simple interest at 10% per year which is payable quarterly, in arrears, beginning June 30, 2013. The Promissory Note may be prepaid until March 31, 2016 at 120% of the outstanding principal balance plus accrued and unpaid interest. The entire principal balance is due and payable on March 31, 2018. The Promissory Note is collateralized by the all the shares of the Borrower and by an investment portfolio consisting of equipment leases, direct hard assets and infrastructure investments. The Promissory Note is convertible, at the Partnership&amp;#8217;s option, into Units of the Borrower, as defined in the agreements.&lt;/font&gt;&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>Carrying value as of the balance sheet date of the Convertible Promissory Note Receivable. Convertible Promissory Note Receivable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the holder.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Convertible Promissory Note</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>Convertible Promissory Note</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://sqncapital.com/role/ConvertiblePromissoryNote</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
