"I, Koichi, am not Jotaro Kujo, only he amogus, can keep up with light speed at this point in time from my personal experience, as such, me, Koichi, am not fast enough to do so" - koichi probably
Also he struggled to keep track of him at a distance, it becomes less difficult to follow objects moving at a distance and even then he was struggling to follow it from meters away while he was covering 3ish meters each. That's generously relativistic. Not the speed of light.
Of course, Act 3 does upscale off Koichi, you ain't wrong, but that's what "at least" is for, we don't have anything even close enough to hard confirm Act 3 is sol or FTL, like, he COULD be, but he could also just be rel given upscaling off a barely rel dude and being just higher rel is entirely doable.
The only way Part 4 is becoming FTL, is if someone reacts to Josuke's restoration post RHCP, while at max speed, unfortunately, that does not occur and the only real time that couldve happened was with his homing bullet, both times Kira failed to react so uh