Did you make sure you are getting the full 5v from pin 44 after sending M106 S255 and then connected pin 44 & ground to the TTL connector on the board? Then apply power to the RAMPS and laser driver, adjusting the current first down and then up just until you see some voltage reading on the laser output. From there I assume you are adjusting the voltage up and it will not go past 4.12v to 5.5v? I am far from an expert, but if you followed all of the steps to that point, it could be a bad board. I say that because the first one I purchased was bad, but exhibited a different issue. I was able to get this far, but once I added the test load and was trying to adjust to 1.7v/1.7 amps it refused to go past 1.25v. Bought another cheap board and it was fine. Just a thought!