@oshizu, so far TS4 has been much more stable than TS3 was. In TS3, while working on a 10-generation game, I had to move my family twice.
Basically, you might want to move to a new save file if you start experiencing problems that "repair game" won't fix, and that you can't put up with. To take an example, the refrigerator glitch, where you can't get food out of the refrigerator and your sims can't cook, can be fixed by repairing your game in Origin most of the time. It's not a serious problem if all your sims need to do is eat -- they can still get quick meals and cook on the grill. But if you need to make birthday cakes and you have a sim with the Master Chef aspiration, then it is a serious problem.
Crashing is another scenario where you might want to start a new file. If your game crashes after installing a new game pack, I'd say wait for the patch, but if it starts crashing for no discernible reason, then your file is probably too large.
At this point, I don't really know how large a file can get -- on my computer -- before it becomes unstable. Once I find out the upper limits, I'll let people know.