Terminator 4 will be based on a sceenplay by Michael Ferris and John Brancato. These are two of the guys who co-wrote Terminator 3: Rise of the Machines, which while not nearly as awesome as the first two films, was still pretty good. I give the writers credit for trying to connect the past and future threads from the previous two films, so that doesn't bode too badly for future projects. On the other hand, this pair wrote the screenplay for Catwoman... (doh!)
The plan for T4 is for it to be the first film in a new trilogy presumably covering what happens between the end of T3 and the beginning of the original Terminator (John Connor sending soldier Kyle Reese into the past). Target release date is the first half of 2009, which although it sounds odd, would make it the shortest amount of time between Terminator movies at six years since the last one (there were 7 years between the orginal and T2, and 12 years between T2 & T3).
Moritz Borman is set to produce the upcoming film. He was one of 14(!) producers on Terminator 3. His comments regarding the upcoming film(s):
"With T3, we included many incidental details and plot points that, along with the main narrative, set the stage for an entirely new set of inter-related stories covering the future adventures of John Connor and the Terminators. This new Terminator trilogy will build upon the already huge worldwide Terminator fan base, which was both revitalized and expanded with the global success of T3."
Sounds like there will be different actors portraying established roles plus new characters introduced as well. It's too early to tell or make judgements on this, so I'll give them the benefit of the doubt for a while. :-)