I got a 4th axis bundled my 3040 machine. I find that mechanically it's not bad at all bad (reasonable bearings and tailstock) , but it's seriously let down by the puniest stepper I've ever seen and a tommy bar chuck that's really awkward to use insomuch as it's not very well built and runs of true by varying amounts dependant on how much you tighten it. It might be worth a look if you already have a chuck and a stepper kicking about that you can press into service and would definitely well enough for you to get the hang of using 4 axiz on very light duty objects but for me that's about their limit.

Col.