This movie seems to be a lot less grueling than doing a Terminator movie. Plus the new Terminator movies are going to be a trilogy that is meant to stand on their own. They wanted a cast that would be able to put the necessary time into filming all 3 movies.
Within the television series "The Sarah Connor Chronicles" they show other human looking terminators. As hybrix mentioned T4 is supposed to be right when the war starts so the endo skeleton models are not built yet. From what I heard they will be the larger robots and military designed ones from T3. Also in T3 mentions that he is sent to kill Connor in the future because of his childhood expereinces with the Governator so in theory that particular skin design could have only been those missions as the humans send back the exact model that kills Connor. They covered the bases.