I am pretty sure that when I was a kidv4 year olds rode tricycles. I think we are just getting a bit silly with our expectations. That said ds7 had a wooden running bike at 2.5 and switched to a 16 inch bike without training wheels on his fourth birthday. Ds5 refused to switch from his plastic 3 wheeled bike until 3.5 and isn't really interested in his bike but will ride a running bike.

I prefer scooters for city kids anyway. They are easy to carry and store and they can stay on the footpath.