To what does the "4" in FT4 refer?

FT4 is a specialized digital mode designed for rapid exchanges, making it highly effective for contesting. The "4" in FT4 stands for the modulation technique it uses: four-tone continuous-phase frequency shift keying (CPFSK). This means the software shifts between four distinct audio tones to encode and transmit data. The continuous phase ensures that the transitions between these tones are smooth, which keeps the transmitted signal's bandwidth as narrow as possible. It does not refer to bits of user information, nor does it refer to transmit/receive cycles. In fact, FT4 uses a rapid 7.5-second transmit/receive cycle, meaning there are actually eight cycles per minute, not four.