Why In the FT-family modes designed by Joe Taylor K1JT and colleagues, the number names the number of tones used by the modulation. FT4 sends four-tone continuous-phase (Gaussian-shaped) frequency shift keying, just as FT8 uses eight tones. The tone count sets the occupied bandwidth and the number of bits carried per symbol.
Watch out The timing answer is the trap: FT4 uses 7.5-second transmit/receive sequences, which is eight cycles per minute, while it is FT8 with its 15-second sequences that gives four cycles per minute.
The digit counts tones, not seconds: FT4 = 4 tones, FT8 = 8 tones.