I have a Chinese 3040 with a 4th axis and maybe I can help you. Do you have the A axis set in Gen. Config. as Linear? I use Linear when engraving on a dia. with the G51 and A axis scale number at the beginning of the file to scale the movements from an engraving code. Example, if your wood is 4" Dia. and you want the image 3/4 the way around it, then multiply 4 X 3.14 = 12.56 X .75= 9.42. Your engraving and Gcode should be generated like it's flat 9.42" the way it will be wrapped. Then you input in the beginning of the file a G51 A 28.662 (270 (3/4 of 360) degrees divided by 9.42), it will then engrave 3/4 of the way around a 4" dia.
Try using the < & > on the keyboard to manually rotate it in both directions instead. If you have the belt drive rotary, and your ratios from the stepper to the chuck are the same as mine, then the steps per. for the A axis should be 20, Velocity 7500 and accel. 250. A axis feedrates need to be set hi in your gcode program, or it will run real slow when rotating. How I keep the other axis's from running real fast also, I set the Velocity max in Motor Tuning for those axis's at the feedrate I want to engrave at. Setup another profile for Mach3, just for doing rotary engraving.
I'm sure there are other ways to do this, but I hope this helps you get started.
Jeff