E.T.
My kids could not pedal a tricycle or a bike with training wheels, so we bought balance bikes instead and then switched to a regular bike with pedals (but no training wheels) at age 5. They've done awesome ever since. Sometimes the problem is the bicycle, not the child.
At age 4, they're still learning a lot of motor control things -- like holding a pencil. Ask his teacher to keep at it and he'll be doing it the "right way" in no time.