It seems like to me that the most difficult part of this deal would be programming the decision making part of the channel letters into mach 4. Going from "I want the letter F in this font and size" over to Gcode is likely where all their custom programming is. Mach 4 can easily handle the motion control and I/O (probably even the encoder error checking), but writing scripts to interpret a simple customer facing graphic interface where the letter, font, size, and quantity can be input would be a tremendous task.
If there is already other software doing this, then it seems like Mach 4 would be a perfectly capable candidate. If not, Mach 4 likely still has more than enough capability, but it will not be an overnight conversion and would require someone with intimate knowledge of LUA to program it.