I have a little stepper + belt drive 4th that came with my machine. It's rubbish. To make it work I need to run stupidly light cuts that are carefully planned to not be too far off centre, because there's nowhere near enough holding or drive torque to do big stuff out on the edges of even smaller work than you're talking about.
Most guys who are having any joy with this seem to be running much higher reductions (50:1 or more), with worm gears (but backlash), harmonic drives (but dollars) or even hypercycloidal drives. Yeah, it means you're not going to be using your fourth as a lathe spindle - or doing particularly fast rapids on the rotaries. But at least it is usable within a sane price range.
Go digging, there's a few threads on here about DIY 4th axis setups which talk about the pitfalls of various approaches.