i want to get source code of UE4, and i have account of git-hub, then how can i check out it?
You can follow the tutorial at the bottom half of this page for getting Unreal Tournament source code with Github. Its the same process for Unreal Engine, just clone the Unreal Engine repo with the Windows Github client.
You can follow this Video tutorial from UnrealEngine on Youtube here
it walks you through the process step by step