Epson 4000 - Manual (Page 47)

Note
You can use the commands listed above while you are
running a program. However, if the program uses one of
these commands for another function, you cannot use it to
change the processor speed.
The speed setting remains in effect until you do the following:
Reset your computer
Turn off your computer
Change the speed with another keyboard command
Change the jumper setting of J22 (as described in
Chapter 4).
Using Your Computer 3-7