4th Axis upgrades are tricky, I've been researching it on and off for a while now. You can find 4th axes on ebay for under $150. The problem is accuracy. With a rotational axis you need a fairly high amount of accuracy. If you are rotating one way, then another, and there is backlash you can start offsetting the work piece. Since grbl doesn't support a 4th axis you'd have to go with Mach3/LinuxCNC (which means no more Easel for running your gcode). Assuming you have the hardware to run those, adding a 4th axis is a matter of getting the axis itself, a stepper, and any old single stepper driver, like a leadshine DSP controller. Also Z-axis height is a major concern. X-Carve has ~2"-3" of travel, which means you'd have to get a 4th axis with a maximum work piece radius of ~1"-1.5", because the center needs to be at most that far off to accomidate the work. I was thinking the other day it would be possible to make the Z-Axis travel higher with a longer makerslide, but you're still limited by the x-carriage height. You could still cut the 4th axis though, assuming the work piece has a small diameter.