Wellll... good grief.
All I'll say is that even if my very young child were way interested in something and crazy talented, I'd certainly let him get his jollies out and I'd nurture him along--but I get the point about making sure that's not his only focus. We pianists know the amount of work it takes to learn, memorize and fine-tune these pieces--what a feat for a wee'un! As well, lots of musical geniuses were/are straight-up crazy, poorly adjusted, socially inept and/or miserable. Genius often comes at a price; there is an argument to be made for balance--regardless a child's natural inclination.
Aside from all that, is it me or does he look like he might be a little older than 4?