Well, I haven't played the fourth game yet obviously, but I think it's safe to say that it will be much of the same as the first three. The series only refines itself with each game really.
Waiting for IV would probably be fine. There's not much of a story to the games, so you don't need to worry about missing anything. I know Japan got a demo, AtlusUSA hasn't said anything about if they will bring that over but it could be good to look for if they do. I don't think there will be any problem in waiting for IV.
Of course you might be able to find one of the older games for less than IV at a game shop. I have completed III and am playing through II, if you wanted to look for one to try out I would recommended III right now, if only for little things like battle speed settings(you can hold down A to speed things up in at least II, but it's not the same) and auto walk arrows you can place on the map. I remember hearing that Atlus will be doing a reprint on the three, but I don't think that has started yet.