I have an Industrial Hobbies mill, and am looking at putting a rotary table on it as a 4th axis. With a stock support and all, would the mill hold the same tolerances as a lathe? Or is the 4th axis setup mostly just for roughing? I guess, what I'm asking is, what kind of tolerances do you guys get with your 4th axis?

I guess it depends on how you plan on using it. Your not going to get the same RPMs as a lathe, so it might be SLOW. Do you plan on running lathe type tooling, if so, you should expect the same type of tolerances that your mill is giving you now. You can always mill/turn it, though I've found the finishes aren't wonderful by any stretch of the imagination.

I wouldn't say a 4th axis is for roughing only, actually I wouldn't say that a 4th axis is very good for roughing at all. Really handy for shaft work with cross holes and flats at specified angles, safety wire holes, and the fun part taking something that would be 4 or 5 setups with only 3 axis, but one with a fourth.