A very athletic 3.5 year old can go straight to a two wheeled scooter and then to a bike without training wheels (the scooter will train the balance).
Even if the balance does not come fast, they can still enjoy a two wheel scooter without balancing. My friend's son enjoys riding our Razors at age 6 and he can't really balance them, though my kids could do so very early. He still enjoys it though and can get pretty fast with one foot hung off to the side to catch himself.
I'd get something like this. Certainly at 42" she is tall enough. I have seen these IRL and my son rode one in the park a year ago. He's 42" now. A scooter like this will also be fun for years.http://www.amazon.com/Kent-Super-Sco...8362839&sr=1-6