is ue4’s editor opensource? if yes where to get it?
If you are looking to get the source code to make your own changes you need to go into your account settings and choose link to github. You will get an email inviting you to the repository where the source code for all engine builds resides. I hope that is your answer.
Adding to this, technically UE4 is not open source even though it is free. Open Source implies the code can be modified and redistributed. The engine '(and editor) is copyrighted.