To what does the "4" in FT4 refer?

Объяснение

The 4 in FT4 is the number of tones: the mode uses four distinct FSK tones and Gaussian pulse shaping to keep the spectrum narrow. The timing distractor is ambiguous rather than merely wrong, because the official documentation counts 7.5 second T/R sequences, eight per minute, while a full transmit-plus-receive round takes 15 seconds; the key follows the reading in which a sequence is a cycle, and under either reading the 4 names the tone count, not the timing.