I'd "just back off" personally. We had so many fewer messes to clean up when our son (also 4) was still interested in sitting down to go to the bathroom.
I don't know if he became interested by watching the kids in day care or if he thought it was a cool new trick because we let him "sprinkle the trees" one day at the park when we weren't going to make it to a public bathroom in time.
My husband hates the "splash" of a urinal and will choose to sit in a stall anytime it's an option - wouldn't it be nice if we had one, too?
He's 4,