Behind the scenes...
The Compositor plugin renders out a series of passes from ZBrush and then puts them together in Marmoset Toolbag 4 to create the 3D effect. This process can be done manually by rendering the different passes or images one at a time. For instance you can just select the Normal Map material in ZBrush (make sure polypaint is off if you have any) and render it. The resulting image can be used as a normal map in the 'surface' slot from a material in Marmoset:
The same thing goes for any other map that you might need for PBR worflows. You could use the 'Compute' button from the Ambient Occlusion plugin and generate a mask to then render out the AO pass with a shadeless material:
This is just a reference so you have an idea of what the plugin is actually doing, but obviously is a lot faster to click a button and let the plugin do all the setup for you ;)
Intro to the Compositor plugin
This is a video series introducing you to the ZBrush Compositor plugin to render ZBrush sculpts with Marmoset and/or 3D Painter
Check it out
