I think TS4 will expand the seamless neighborhood concept into a seamless country or even a seamless planet. Then with TS5, a seamless galaxy (lol Spore)! Seems like a natural direction to go in. With all the debate on how TS3 has gotten rid of most of the Sims charm I wonder if EA will make TS4's graphics even more realistic or pull a Legend of Zelda Wind Waker.
I'm not ready to look forward to TS4 just yet though, especially if they make us wait another year or two after the base game before we can have weather and pets in the game yet again. I know those things are hard to program but it's going feel silly going back to an always summer and petless (and therefore almost lifeless) world again. As it is with TS3 I'm praying weather and pets are done with soon so I can start wishing for brand new stuff instead. Hopefully TS4 base game has at least one of the two, I'm thankful TS3 at least included cars, gardening, and fishing in the base game. And since those were included with the base game they were able to be expanded on more as more EPs come out.