It's not overly busy here, but busy enough. There are always some people around.
While not everyone here tried TS4, most have. Some play it regularly or both games (or even TS2), others just TS3. Given TS4 has been given away for free a few times, most people have at least looked at it.
How easy it will be to get your old stuff back is anyone's guess. The experiences differ wildly.
I think TS4 may need a 64-bit system, but it doesn't use as many resources, at least as long as you don't load all DLC. So the answer is: depends very much on your system.