Do you mean a max limitation per USB device? Not a total of 4 axis for all devices combined.
I keep thinking the pedals should be able to register as a 3 or 4 axis device, with X, Y, and Z axis' that can be calibrated in Windows. Two for the toe brakes, one for the rudder.
If the pedals are connected alone to the USB adaptor you can try and see if you can set it up as a 4 axis controller but if there is another device connected to the adaptor along with the pedals, you won't get the toe brakes to work since the max axis recognized by most adaptors is 4 axis.
A problem that cropped up many years ago when I was at CH and testing the Rockfire USB game port adaptor with a set of analog Pro Pedals was the calibration process of the toe brakes. I had to jerry rig a button to the pedals to allow me to calibrate properly since for Windows calibration, a button press was needed to save the calibration (pressing Next when calibrating in Windows does not save the calibration, it just skips that calibration step) when I was testing the adaptor with just the pedals and no joystick connected.
ack-ack