SD4 with Rumba only prints at back right corner

I had the exact same issue after getting my RUMBA up and running on my SD2, so I am happy that I can actually help someone instead of being the one asking for help for once!  Check out my post under hacks and mods. 

I don't know why it does that, but I did find a work around.  Go under your printer shape in repetier.  The bed left and bed front fields under printer settings will take negative values, so I took a guess and set them each to -40 and printed a 20mm calibration cube.  Doing a little math we can see that the edges of the cube should be 65mm from the edge of the bed.  After printing but before removing the part from the bed, I took out the calipers and measured the actual distance that I was getting.  Doing a little more math, I found that setting both of those fields to -57 results in the cube printing in damn near the middle of the bed (I centered the print).

Since the SD4 bed is 50 mm bigger, I would try that test with your values set to -107 and see if that calibration cube lands in the center of your bed.  Like I said, I don't know why it does that, but I just found a work around for now.  The downside is that under manual controls, where it shows your object being created in real time, it will only show the part of the bed that is >0, so you essentially lose the cool graphic of watching the part being built on your screen.

Let me know if you find a better way to fix it.

Awesome, it fixed the problem, though it does sort of eliminate the usefulness of absolute coordinates.  I'm happy though, I just center most of my prints anyways.  Thanks a lot!