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401(K) Plan - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020
2014 401(k) Plan |  |
Defined Contribution Plan Disclosure [Line Items] |  |
Employer contribution | $ 0.5 | $ 0.4

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

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_RetirementPlanNameAxis=fdmt_TwoThousandFourteenPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |