@mrjadkowski
I loaded V4 from here: http://firmware.eu.ardupilot.org/Plane/latest/PX4/
Has the Pixracer a Memory Card?
May be, thats the reason.
If not, I uploaded the file here: https://www.hightail.com/download/dDZIRFFYT2J6RTk3czlVag
For me, it works.