There is really zero reason to expose any 4 year old to any other gaming platform than an iPad IMO. It’s so damn good at that, pretty much the platonic ideal of a small kid game device.
(Also I recommend the big grips case, should fit the standard 2017 iPad fine, I’d think, but not the pro. Obviously don’t get your 4 year old a pro. I made that mistake one and never again.)
https://www.amazon.com/Big-Grips-Frame-iPad-Blue/dp/B005641NC4
As an older kid, sure, begin to widen the exposure…