GTK4 can’t be used in Phosh in its current shape - although things appear to be progressing upstream to make it possible. See https://gitlab.gnome.org/GNOME/gtk/-/issues/2132
That said, there’s no huge benefit to reap performance-wise just by porting Phosh to GTK4. Phosh doesn’t render that much itself, so you’ll get a smoother app list scrolling and that’s pretty much it. However, GTK4 will allow to easily implement some stuff that we simply didn’t bother implementing with GTK3 because it just makes it so much harder to do in a reasonable and performant way (an example from what I personally worked on would be live previews in the app switcher).
What most users actually want to happen in order to make the whole experience smoother is applications using GTK4 (and GTK4 being performant on mobile GPUs, which wasn’t the case so far but there’s some work on it ongoing as well).