crazy question, why did you get Mach2? Mach3 is much better.......
at any rate.
Your questions
1). Screen 4 can design for Mach2
2). You can't directly password protect your screens like you want, BUT..... You can do it indirectly
In screen 4 put a image button on the screen, put image as none, on the Hotkeys button tick it off and hit some key combination like Alt-w or what ever, and shrink the button down to its smallest size, then hide it under a company bitmap logo in the bottom right corner or something, put no other buttons around the logo. The chances would be real low that an operator would be randomly cliking on a logo for no reason. BUT, when you need to goto the other set up pages you just hit the hot key combo.
I have attached a screen set example done as above for your review, I have called the screen set "Hidden", copy and paste it into the Mach2/3 main directory, open Mach then hit view, then load screens, then navigate to the "Hidden" screen and hit ok, play with it, but before you close mach, reload your old screen back in place OEM is 1024.
NOTE: If for some reason your user is drifting around the mouse all over your Logo, if he goes over the "Hidden" button, his mouse will turn to a hand, if he pushes it, it will goto the other page, thus why I say put the logo as a larger object off by it self with no buttons around it.
Scott