Well my son has not long turned 4 so i recommend a "real" bike as soon as they can manage one. He's been having the real wheel lack of traction thing too - and sometimes he's pedalling and he's giving it too much, the chain slips and the wheel just whirrs because he's going faster and harder than the little bike can manage.

(remembered what his new one is called - it's a 16" raleigh Zapp)