Unless the wireless has been giving you problems, then you should be able to leave it be, turn on Moca networking in the xl4, and the mini will only need a coax connection for full functionality.
If you have the ability to do so, wired Ethernet to the XL4 would be a more reliable connection with likely performance advantages over wireless.
Depending on your layout/wiring, another choice is a Moca adapter that could be used as a bridge to create the Moca network, and then both the xl4 and mini would only need a coax connection.
-Roamio Pro *Just out of the box!* :D
-Premier Xl4 MOCA bridge w/Slide Pro
-Mini Lifetime MOCA
-Comcast/TiVo Mutant dvrs - RIP
-2x Upgraded Series 2 - Retired