4F
4F or 4-F may refer to:
- 4F (music), key signature of four flats
- 4F correlator, or 4-f system, a Fourier optics system for image processing
- Flottille 4F a French naval aviation squadron
- LMS Fowler Class 4F, a class of 0-6-0 steam locomotive
- The 4-F classification in the United States' Selective Service System, identifying a person as unfit for military service after formal examination by the Entrance Processing Command
See also
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