I didn’t need a powered usb for my wireless adapter. But I am running pi 2, not sure what your setup is.

I used option 3 from here https://github.com/retropie/RetroPie-Setup/wiki/Setting-up-the-XBox360-controller

Then I had to make edits to the rc.local file in order to pair player 3 and 4. Like I said I can go into more detail later in the week