The way we did it with my son when he was like 4 was to do both ways and then merge them.

1. He learned to ride a balance bike.

2. He learned to ride a bike with training wheels.

3. I took off the training wheels, then had him practice, without actually pedalling around yet, catching himself with his feet. He would place his feet on the pedals while I held the bike upright, than I would let go of the bike and he'd move his feet to the ground to stop himself from toppling over.

4. Once he was comfortable catching himself, he was able to start pedalling around for real, easy peasy, because he was confident in his ability to stop himself from falling if necessary.