Neither of them (Mako/Bolin) are 'air' benders though (they are fire/earth respectively).As for who you'll play in Book 4, I'm 100% sure that it will be the cute brother (I forget his name lol) for both routes. It makes a lot of sense since he's on the pro-bending team, lives at the stadium, and meets/dates the rich girl who will also have scenes.
My money is still on a random air bender for one route, and Zaheer for the other. That said, Bumi or Opal would be interesting options...