You can change tools now by just adding T0 or T1 to the Gcode.
On toolchange the Gcode enters absolute mode (G90), move to other tool and then back to relativ mode (G91).
This way the calculations of cartesian to bipolar doesn't have to change and 
additional tools can be added fairly easily. 
With 'only' 4 tools you get the benefit that no tool overlaps with the built area of the other tools.
So you need no separate z-axis for every tool.

And some goofy distortion when starting off-center: