I've owned two G4000s, and converted both to AXA QCTPs. First off, you need an adapter sleeve for the fixed stud atop the compound slide. Little Machine Shop sells one:
QCTP Accessories 3712 Makes AXA QCTP a bolt-on accessory for 9x20 lathes; Threads over standard tool post stud; Outside thread is 9/16"-18; This tool ...
littlemachineshop.com
I've used both piston type and wedge type QCTPs. Both have worked well. Shars, CDCO, LMS, etc. sell the sets, as well as additional tool holders. I've not had trouble with any of them.
Possible modifications:
1. I noticed in at least one case that the stress of tightening the mounting nut tended to slightly bulge the center of the compound. When that happened, and the QCTP was mainly supported near the center, it became a bit too easy to rotat under load. I fixed this by chucking the QCTP in a 4-jaw and cutting away the central area about .010" deep. This shifted the holding force to the periphery of the QCTP head and held it firmly against unwanted rotation. PS - that's the LMS adapter stud on the compound.
View attachment 344734
2. It seemed like the operating handle on the QCTP was not well located - always getting in the way of things. So I drilled/tapped three more holes at 90º intervals (using my mill). One of them turned out to be the best, so I left the handle there.
View attachment 344735