I only used three drivers, actually. I wired the coils of each motor in series. It’s the same wiring as though you were using one of the Marlin boards.
One stuffer for X, wired in series. One driver for Y wired in series. One driver for Z.
Also, I have a v1pi image now, which has the latest (or near latest) cnc.js installed and some other goodies. It also has octoprint, but that doesn’t work well with grbl AFAIK.
Here’s Ryan’s link:
Ah yes that makes sense. I should have realized that the pi-hat board you had is probably the older version that only has 3 sockets anyways. The latest one on ebay has an A axis socket as well and you can clone either the X,Y, or Z to it by soldering some pads on the backside. This is my first CNC and/or printer build so I wasn’t aware that a single stepper driver can actually control multiple stepper motors. Thanks for the link, I’m sure I would have seen that eventually when I went to do my build. Still printing parts 
In many ways it’s actually better to drive the two motors for an axis from one driver - imagine what happens if one of the steppers fails or overheats and shuts down.
With a single driver, that axis stops moving and you probably ruin your project.
With separate drivers, only one side of the axis stops, the other side keeps moving and it starts twisting and trying to rip the machine apart.
There’s no trying! Either of them can pull themselves apart. I had to reprint parts…