With four motors your first major challenge is going to be to get the thing to run in a straight line, or to run along any course. Do you have this taken care of already?
The next challenge is to plot a course - vortex shape will work fine for a circular garden, but not many are circular. Or even square. Or even just four straight edges. How is that part taking shape?
Then you have to have a way to know where you are. How much distance did you cover? What direction are you pointing in right now? You don't mention any sensors. Just some unspecified keyboard giving three "inputs".
Safety comes in play. Overrunning a little child with a grass cutter can make the child look rather ugly, chopped up and so.