Registrant not qualified for military service
4-F – Registrant not qualified for military service. 4-FM – Medical specialist not qualified for military service. 4-G – Sole surviving son – son or brothers in a family where the parent or sibling died as a result of US military service, or is in a captured or M.I.A. status, are exempt from service in peacetime.
 

https://en.wikipedia.org/wiki/Apartment_3-G 

3-G

2H