I would suggest dropping the evil bit as alphabets generally rapidly spread themselves due to there being relativly easy to learn and write. I would make them more mercenary than evil as they would have substantial long term advantages in being the power behind the crown, due to it being really handy to have someone who can read and write to keep track of pesky things like taxes, armies, and spell libraries.
If you do want to go the secret hidden evil wizard route, you could have a base alphabet that is known by the public through out the 4 nations but a syllabary known only by the wizards that is an out growth of the base language. The wizards promote the use of their language in everyday life including schools, govermetn and business but ruthlessly repress knowlege of the hidden syllabary. The syllabary would make an excellent basis for the usage of Book/Path magic from the Thaumaturgy book, as it rewards both general knowelege of magic principles and specific dedication to set rituals and rites.
http://en.wikipedia.org/wiki/Syllabary