I bought my son a Trek Jet 16" a few weeks ago for his fourth birthday. He's a bit short for his age & as you can see in the pics, there's still room to adjust it lower. He could have rode it at 3 no problem. It is a good quality & a bit spendy for a first bike at $150. I figure you get what you pay for.
Edit: On fitment, I adjusted the seat so the his knee is slightly bent when the pedal is at the lowest point. My son watches Dora, but prefers Go Diego. He wanted a battery powered riding John Deere dump truck like the nieghbor has, but he already has a Corvette. I told him that if he whined anymore about it, the car would be sold & he wouldn't get anything. I'm not going to raise a spoiled brat. I didn't hear about the dump truck again.