Manual says M4 at Min 20mm, Max 25mm. I purchased 22mm M4 screws. However, when screwing them in, it goes in about half way very easily and then gets tight...too tight to screw any further with fingers without a tool. Is this normal? Should it screw in easily for the full 20 to 25mm of the screw? I'm afraid to keep screwing as if too long, could damage the PJ.
It sounds like you are bottoming out on the screws. There is a small metal plate at the end of the screw hole that will stop anyone from pushing the screws into the circuitry.
It should screw in easily.