It appears that although on t.v. Jim Cameron said he was going to do T4, it's listed Currently in Production on the internet movie database
under the direction of Jonathan Mostow who did T3.
Arnold has not been confirmed for the role, their are rumors on the internet movie database that they are casting a new terminator.
It's supposed to come out in 2006 according to that website and they are shooting now.
In other News Star Trek XI or rather star trek 11: The Beginning, is coming out holiday 2007.
It now has the director of Wrath of Khan and The undiscovered country (NIck Meyer) attached to it, and yes it is a prequel of unknowns.
Supposed to be about the wars between Earth and Romulas.
They are just having Nicholas Meyer shoot the damn thing though.
I don't think he has any creative involvement since it is a story by rick berman and Erik Jendresen, maybe he'll have third credit as a story/ script consult but that is probalbly it.
I think this is a ploy to get trek fans back into theaters so they can make money. Just by having meyer involved in any capacity,
since he directed the mother of all star trek films, The wrath of Khan.
Berman tried this before by trying to get Leonard Nimoy to direct Generations but have no creative input, just a well known name in trek fandom to sell the picture.