Yes, you can run another stepper off the 4th port in the G-540. And a rotary table can be quite handy for some projects. Without knowing what you intend to do, it's hard to say whether that thing you found will be sufficient for your needs. But some of those "no name" 4th axis kits on Ebay don't have much (or any) gear reduction, have lots of slop in the bearings, and don't support the 4-jaw chuck you'd need to hold anything but round stock. Buyer beware,,,.