You would only need 3 pins to control the positives. The 4th lead is a ground. It may be the other way around (3 neg and 1 pos) as it's been quite some time from when I last messed with RGB leds. Also, if you use analog outputs, you can vary the color output and brightness.
RGBW is also mentioned, but I'm assuming you are using RGB.
I don't know anything about RGB support in marlin, but I do know you can use GCode to control individual pins, which would seem simpler to me.