Design software > Toolpath creator > Gcode creator > Gcode loader > Firmware > Hardware control board
The standard firmware for the MPCNC and LowRider is Marlin. It’s core was developed as 3D printer firmware that has CNC functionality added to it among other things. It does not have a fourth axis functionality. I agree with @jeffeb3 that LinuxCNC might be what you are looking for in terms of control. @dkj4linux has some some cools stuff with it.
The Synthetos tinyG board has a lot of folks doing interesting stuff with it in different axes. You can use ChiliPeppr to control the board or other firmware solutions.
If you aren’t doing huge tangential forces on the tool and only have a minimum Z height, as you might be working with leather or paper, then a larger MPCNC wouldn’t be too bad because the side load is minimal.
Just thinking out load. I have played with LinuxCNC and the tinyG and both have lots of possibilities. Then again, depending on what you are doing, the standard Marlin firmware and usual hardware might be enough for your needs. What materials?
And welcome to the forum @beapilot