I have just started to use Lightburn with a newer version of GRBL intended for the Mega 2560 board plus or minus the RAMPS shield. It is version 1.1L and is intended for true 4th (and 5th and 6th) axis movements. It is available on Github. Search for GRBL Mega 5X. It has Laser mode of course. It can easily be loaded with Arduino IDE or with the Hex file and XLoader.
In the past I had some success mapping a curved surface and burning to that with a ‘fake’ 4th axis using one of my Uno linear axes as a rotary axis. I would tell Lightburn to use Y as the rotary for example.
Using the Mega 2560 board, I have set up the A axis in GRBL under $133 and $134 and calculated the proper number of steps for my regular stepper direct driving my 4th axis - 200 steps, 32 microsteps divided by 360 degrees results in a ‘number’ for A of 17.778 steps (per degree I think). At any rate the piece moves accurately with G1 A___ F___ where A is in degrees. It looks like the prior GCode I created with Lightburn is moving things properly.
It looks like Lightburn will handle this properly as a 360 degree movement device although in setup when I select A axis the 360 is greyed out and the settings is still looking for object diameter and/or circumference. I am wondering is this is just to help scale the image to the object and the 360 is defaulted? Is this a true use of the 4th axis? Are A commands to GCode properly created?