I'm playing through as a pure Mage, and I currently only have 3 skills mapped. However there are 7 skills in my abilities tree but only 4 available buttons: holding RT and then A, B, X or Y. If I were to purchase all 7 skills, how would they be mapped on the controller?

You can't... because the game's like something out of a by gone era for RPG's =/ (and even then like in something like Oblivion, there was always the wheel). It's fucking lunacy as to why you can only store 4 spells despite they of course offering up quite a few across the three trees. You can at least activate a sustained spell, then remove it from your set and replace it with an active spell but it's still a complete joke.

Wow. I guess this is just more of a reason for me to focus on four, I guess.

Yeah, and as far as I can tell the only purpose of the d-pad is a quick health potion drink. I like that feature, but couldn't I have four more spell selections on there? Course, at this point I just like seeing my giant meteor go boom.

I'm pretty sure the PC version allows you to map 10.

Yea I'm playing on PC with a controller and found myself just having to respec around 4 spells with my Might/Sorcery hybrid. Seems kinda weird design choice but the games so easy, you can get the job done with 4 spells anyway.