I see how mp is rather redundant, but just like classes having the names of Gladiator, Pugilist, Thaumaturge, as opposed to Warrior, Monk, and whatever, it feels of. It doesn't change much, but it does.....in a way.
I think they should keep mp, but they should lower how much MP players have, and not all spells should cost mp. Reserve it for the "Ancient" magic.
A fourth bar wouldn't help me, I don't like using more than one. Hell, I'm still disappointed that I can't use the number pad instead of the numbers along the top. It's difficult for me to use anything past six, on a single bar, due to how I move around a lot, sometimes the keyboard is on my lap and my body is facing one way with the television to my right, sometimes I'm laying down, and the such.
In a game that supposedly requires positioning, I like to keep one hand to allow me to move around and they other to use my abilities, so that I can be precise and quick without looking away from the screen.