Retrodeck in your case then if you just want a simple setup.

There’s a docker image called wg-easy I use with a nice web interface.

Armbian with CasaOS

I can’t speak on moonlight specifically on the pi 4, but it running steamlink on wasn’t bad on it, so I can’t imagine it’s would be any worse than that for moonlight.

Wired Nvidia shield TV has been my best experience when it comes to both moonlight and steam link out of all of the devices I’ve tried it with.