I have written extensively about FT8, including how to get it running on the IC-7300 Mark II and what WSJT-X is actually doing under the hood, but FT4 barely gets mentioned even though it ships in the same software and takes zero extra setup to try.
The One Real Difference: Speed
FT8 runs on a 15 second transmit and receive cycle. FT4 compresses that down to 7.5 seconds, which doubles your theoretical QSO rate if band conditions cooperate. That single change is the entire reason the mode exists. It was built by the WSJT-X team specifically with contesting in mind, where working through a pileup faster matters more than squeezing out the last few dB of weak signal performance.
What You Give Up for That Speed
FT4 is meaningfully less sensitive than FT8. It cannot pull as weak a signal out of the noise floor, and it is more sensitive to timing drift and keying issues on your end, since there is simply less time per cycle for the decoder to work with. On a marginal path where FT8 might squeak out a decode, FT4 often will not. That tradeoff is exactly backwards from what most casual DX chasing actually wants, which is why FT8 dominates general activity while FT4 stays a niche mode most people have never tried.
Where FT4 Actually Shines
Dedicated FT4 contests exist specifically because the faster cycle time rewards contest style operating, the same reason RTTY and other faster digital modes hold their own dedicated contest weekends alongside slower FT8 activity. If you already enjoy the contesting side of the hobby I covered in Contesting 101, FT4 is worth having in your back pocket for events built around it. Outside of a contest, the band being quieter on FT4 than FT8 can actually work in your favor too, since there is less congestion and less chance of your own signal getting stepped on by everyone crowded onto the same few kHz.
Trying It Costs You Nothing
If you already run WSJT-X for FT8, switching modes is a dropdown menu, not a new install. There is no reason not to try it during the next FT4 specific contest just to see how the faster pace feels. Most people either love the quicker rhythm or go right back to FT8 within the first session, and either way you will know which camp you are in without spending a dime finding out.