Currently, I have an outdoor house light connected to the same light switch as a couple other garage lights (see sketch). I would like to separate the garage lights from the outside house light using two separate switches. Only thing is, the switch is already in a three-gang box. So, I already went ahead and bought a four-gang and I installed it, but now I'm having trouble figuring out how to wire it all. The way it was is confusing to me and I'll try to post an image of how it currently is. Anyway, the other two switches in the box control a light in the house and another completely separate that isn't causing any issues (dimmer switch for kitchen lights), so you can ignore that. So yea, main goal is to add a fourth switch to the box to separate the garage lights and outdoor house light. Thanks!
I actually managed to figure it out on my own. Referring to my picture, I moved the red wire to its own switch and added another white wire and attached it as well. Problem solved. Only downside was trying to pigtail 4 wires into one.
Few cleanup issues
You need to hook up the grounds. Given the stupid short wires, all these cable runs must be replaced OR go back to a metal box and ground them to the clamps like the last guy.
You got the memo on not double stacking screws on wires, good. You see how roached that looks, anything shabby like that is outlaw, NEC 110.12.
4 or more wires should use a red wire nut. They should survive a pull test. If you need tape to make them stay put, fix your technique instead.