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Statements of Operations (Parenthetical) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020
Income Statement [Abstract] |  |
Research and development expense, related party | $ 935 | $ 579

X
- Definition Research and development expense related party. + References No definition available. + Details
Name: | fdmt_ResearchAndDevelopmentExpenseRelatedParty
Namespace Prefix: | fdmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_IncomeStatementAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration