What is the difference between a two stroke dirtbike and a four stroke?

A 4-stroke will run quieter and cleaner. It will rev lower. For a given engine power it'll be heavier and bigger. For a given engine power it'll use less fuel.

The engine will need more parts and be more complicated

4-strokes generally have better torque at low revs.

A 2-stroke will run noisier and pollute more, It will rev higher, for a given engine power it'll be smaller and lighter. For a given engine power it'll use more fuel. The engine will consist of fewer parts, and be a simpler design.

2-strokes generally have less torque at low revs but do better at high revs.