Why not? Why wouldn’t you use it? Ok, it doesn’t have as good of an SNR as FT8. But it’s a hell of a lot more fun than watching paint dry with FT8. I’ll often jump on an FT8 subband and see strong signals all over the place (good band conditions somewhere). The subband will be full. I’ll switch to FT4 and see no signals.
You can complete an FT4 QSO so much more quickly than an FT8 QSO. This is especially true if you disable TX1 when calling another station. It can become really fast paced on FT4 when the conditions are good and a lot of stations are on. It is actually much more fun than the monotony of FT8.
I’d encourage everyone to switch to the FT4 subband once in a while and see if you find signals active there. If you do, start calling stations. If you don’t see any signals, make a few CQ calls to see if there are any lurkers waiting… there often is.