Guys i'm looking for some advice on a first bike for my boy. His 4 year birthday is coming up and i've been telling him all year he can get a motorcycle as soon as he learns how to ride a pedal bike without training wheels. Well, he's been riding without training wheels for about 6 months now

so I'm pretty much obligated at this point. He's 42" tall and has a 17" inseam. The 2 bikes i have it down to are either the yami pw50 or the ttr50 (crf50 is out because it's red and blue is his color of choice as of late

). My guess is the pw50 would be easier for him to start off on but i'm scared by how quickly he might out grow it. I also don't care much for the rear brake being on the handlebars as that's one more thing that'll change when going to a larger bike.
TL;DR , is a ttr50 an acceptable bike to start off on for a 4yo or am i setting him up for failure? While he can ride a bike for probably hours without training wheels he still can't start off by himself and still doesn't understand the concept of the brake
thanks in advance!