I just reloaded mach 3 to check that very thing, since I have determined M3 was not interfering with 4.
So I think we can assume hardware is plugged and setup correctly, M3 works with keyboard and mouse jogging.
I think it is some switch that M3 sets to a default and M4 needs some input.
I am going to start a fresh profile so if I set something in error or it is corrupt.
I live in the Denver metro area,, and have spare room, adult beverages and 3 mills, 4 lathes a 3 d printer, and a non working router for you to play with.
I may ship it to you in a week for 2cents. I thought i was doing well learning how to make chips, electronics is a whole different animal............ I dread the next project which is building a control box with a smooth stepper to use M4 to get CNC on the mills..
Oh well the weekend is here, lots of time to pull my hair out!
attached are the settings files, if you see anything interesting,