I would say it depends. One of the basic changes made to GTK4 was deprecating (in fact, actually removing) support for window placement. That's fine if you're using a window manager that takes care of it, but because I have yet to find one that does it reliably across multiple displays, I've opted to stick with GTK3.
To sum up:
- if you have no need for window placement: GTK4
- if you need window placement, but only have one monitor: GTK4 (but make sure you're using a DE with a window manager that takes care of placement)
- if you need window placement and have multiple monitors: GTK3 so you can take care of placement yourself in your application