I got the same problem the last day on the workshop computer, it had an old version of Mach3 on it so I downloaded the latest version 1.9 something and installed it. When I next ran screen 4 it worked great.
One thing I have found is if you have screen 4 open with your current screenset and also have Mach open, if you edit the buttons script in Mach (only place it can be edited) all of the LEDs will go back to system function instead of OEM and I have to go back in screen4 and check all the LEDs as OEM. Easily solved by making sure I dont edit script buttons when screen 4 is open

Hood