How do they get 4 stroke engine ondirt bikes?

they just do

Yep, they just do it.... :)

I guess you're wondering how do they get an engine that's in my car, into this small bike. Well although these engines work like the one in your car, they are very different. To start with, there's only ONE cylinder, as opposed to four or eight (or whatever) in your car. And the second big factor is that the engine in you car is at least 1 liter, probobly more. 1 Liter is 1000cc. Dirtbikes are usually up to 450cc. There are some over 700cc but definitely not more than a liter.

So there you go. You have a SMALL four stroke engine, now you can put it wherever you like. Answer If you go back in history a few decades, you'd that most, or nearly all offroad bikes were 4-stroke. They were heavy, and not especially fast, but then few offroad bikes were (fast). 2-strokes were dirty and didn't like running at low speeds as one must when riding offroad- the sparkplugs would foul up. Air-cooled two-strokes were prone to overheating and seizing, as well.