Have you considered using something along the lines of a Blue Mage? Let the player pick three different schools (instead of 4), allowing him to make his own specialization.
I'm not opposed to the idea of a Blue Mage. Strago was one of my favorites from FFVI.
While I had considered having a Blue Mage, I thought it would distract from the nice duality the others have. Also, I did not want to create a mechanic for acquiring spells from monsters fought.
Picking 3 schools (or 9 disciplines) would not be unreasonable for the versatility it could offer, but it's not for my game.
Also, of note, if you rename Red to Blue, and Black to Red, you'd only have to create Black for a 5-color Magic system.