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4F or 4-F may refer to:
- 4F (music), key signature of four flats
- 4-F (US military), a military classification under the United States' Selective Service System for candidates found to be unfit for military service after formal examination by the Entrance Processing Command
- 4F correlator, or 4-f system, a Fourier optical system for image processing
- LMS Fowler Class 4F, a class of 0-6-0 steam locomotive
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