Find a shallow grass slope, preferably leading onto flat area. If the slope has varying gradients along its length, so much the better.
Remove pedals (rh thread on rh pedal, lh thread on lh pedal) and lower seat so h can get his feet comfortably on the ground.
Let him (encourage him) to roll down the slope with his feet down. He should soon get the hang of balancing and steering. Once he gets comfortable with the length of slope you start him off on, take him further up the slope, or on a steeper part of it.
He'll soon be able to control his steering and ride in a fairly straight line.
Then put the pedals back on and he'll be redy to start pedalling on his own. With a little judicious help from dad.
Tip: if you have to push him to get him started, push him, not the bike, otherwise the bike will go where you push it, which may not quite be where he is steering it.
If, at 4, he's not quite ready, the scooter is and excellent idea
Alternatively, if he doesn't like the slope idea, just leave the pedals off and keep the saddle low enough for him to be able to "walk" the bike along with his feet. This also works and he should get the knack of steering with his feet off the ground.