Try removing the _btn from both the entries, as per comments in the retroarch.cfg file:

# Saves state.
# input_save_state = f2
# Loads state.
# input_load_state = f4

If that doesn’t work still in USBGamepad.cfg, try adding it back into retroarch.cfg as well.. couldn’t hurt at this point =P

I have not actually tried a Save/Load state on my end yet.. still hung up over this 2 player issue heh. I’ll try tomorrow and see whether mine works.