Number of warrant shares issued.

No definition available.

Name:	fdmt_NumberOfWarrantSharesIssued
Namespace Prefix:	fdmt_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Warrant expense.

+ References

No definition available.

+ Details

Name:	fdmt_WarrantExpense
Namespace Prefix:	fdmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Warrant expires year.

+ References

No definition available.

+ Details

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Namespace Prefix:	fdmt_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Warrant vesting period.

+ References

No definition available.

+ Details

Name:	fdmt_WarrantVestingPeriod
Namespace Prefix:	fdmt_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-3

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ClassOfWarrantOrRightLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

+ Details

Name:	us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash inflow from issuance of rights to purchase common shares at predetermined price (usually issued together with corporate debt).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 14
-Subparagraph (a)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-14

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockPricePerShare
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_CommonStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=fdmt_PreFundedWarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Common Stock Warrants - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	Dec. 31, 2020	May 31, 2018	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Feb. 29, 2024
Class Of Warrant Or Right [Line Items]
Number of warrant shares issued	30,000	23,669
Exercise price	$ 18	$ 3.19
Warrant expiration year	2027	2025
Warrant vesting period	4 years
Warrant expense			$ 0	$ 0	$ 0	$ 0
2024 Follow On Public Offering \| Common Stock
Class Of Warrant Or Right [Line Items]
Warrant shares become exercisable							3,583,476
2024 Follow On Public Offering \| Common Stock \| Pre-funded Warrants
Class Of Warrant Or Right [Line Items]
Exercise price							$ 0.0001
Warrant shares become exercisable							3,583,476
Common stock offering price							$ 29.4999
Issuance of pre-funded warrants					$ 99,400,000