@john0945 Welcome to the group.
It is my understanding that grbl cannot accommodate 4 axes, so Sienci did a work around for its Vortex by both creating a post processor for Vectric that translates Y moves into A moves, and offering a Y/A switch.
As you have seen grblHal can accommodate 4 axes. There has been a lot of talk on here about that. Here is one link gSender - Hal + Rotary support Early Alpha
This one is quite old, but I believe that the basics are still valid. I know nothing about FluidNC, so this link or others you will find with a search may not address your issue directly.
If they do not, you may want to add some details to your question, such as the CAM application you are using to generate the rotary toolpaths and the post processor you are using to generate the gcode.