Xr 50 And Cr 250?

I bought a XR50 for my 5-year old daughter. If your 4-year old can ride a bike without training wheels, he will do fine on the XR50. Just make sure you set the throttle stop to 1/8 throttle and start him out in first gear so you can trot behind him until he gets used to it. Then increase the throttle stop to 1/4, then 1/2. Then put it in second gear and let him get used to that before you adjust the throttle higher.
The only problem is that the shifter is too hard for them to shift up on, but the little tykes can start out in 3rd gear without much problem, and I just taught her to shift down for hills.

A CR250 will feel like it has no low end and you will be screaming it and shifting alot more compared to your 500. Get a leftover new 500 from some dealer somewhere, or ante up for a CR500af from Service Honda.
My brother has a CRF450, and it is pretty fast and has a nice wide powerband. It is faster than most 250s, and you don't have to scream it and shift like a madman. It is faster than it seems because it gets better traction and has less wheelspin than a CR500. But when there is traction, nothing beats the arm-stretching of a 2 stroke 500.

You could always get 2 xr50's and make one an adult xr50!

4 is definetley NOT too young to start riding. I started my son a week after turning 4. He flies now at almost 7. 25' doubles and all. My 4 year old daughter raced her Pw until she turned 5 now she races a Cobra50. The Xr50 is a great!!!! bike. I think I had more fun on it than the kids though. lots of hop up stuff too. never any problems mechanically other than clogged air filter...yeah I kinda just forgot about working on that bike.
one thing we did was tie a rope to the back of the bike for my kids. when they started going too fasy I'd tug a bit to slow them down. May work for you?? We never had the throttle stop screw in place on the PW but we did have the exhaust restrictor. Not sure if your in to racing but their are a lot less options for racing the Xr than the PW. If you just want to cruise around then the xR50 is a much better bike in my opinion.
Jason

One thing that might be worth at least thinking about. You don't want to kill your boy with the roost

I think LOST is onto something! Get a XR80 for yourself and when the kids are older they can grow right into it! A bigger bike for working with young kids just gets in the way! 2 strokes are especially not real get at stop/start playing needs that happen when following junior. I would really consider this thought - I did (I rode a XL100 when playing with the kids) and have NO regrets (course I also had my own CR500 in the stable for serious play). The XR50 is great for the kids and 4 is NOT to young to start - just follow the previous suggestions and you will be fine.
Woodsy
An added benifit is that your wife will be thrilled seeing you slow down to spend time with the kids - trust me!!!!