My 4 year old (turns 5 next month) learned how to ride her bike over the course of 3 nights and ~6 hours. I used her old 14 inch bike as a coaster bike (no peddles). She mastered coasting w/o putting her feet down for a whole block (we live on a slight hill) so I put the peddles back on it. By the end of the second night she could peddle her 14 inch bike up and back where ever she wanted. Roughly 20 minutes into night 3 she figured out how to peddle in a circle, and by the end of tonight (night three) she was riding her Giant Puddin' just as well as her much lighter and smaller Lil' Pudding.
It was a great night!