Switched the hardcoded references from 4 to 8, so that the tool can support up to 8 players.

[MeltyPlayer]

One of the issues that's personally blocking me in my RetroPie setup is not being able to map more than 4 joysticks. I have kind of an odd setup where my joysticks are internally listed as 8 different devices, because my USB encoder buttons are separate from the actual arcade sticks.

I noticed that there's an existing ticket for this exact issue, so I thought I might try tossing my hat into the ring to fix it: https://github.com/meleu/RetroPie-joystick-selection/issues/18

This joystick-selection script is currently hardcoded for 4-player support; I set up a barebones commit that just changes this to 8-player support instead. There are likely at least a couple MAME games that would require more than 4 players (e.g. the X-Men game supports 6 players), so this didn't seem too unreasonable of a change. But please let me know if you have any concerns that I should address.