I originally purchased the JT 4000 as a travel synth. But I was frustrated that despite studying various YouTube videos I entirely failed to be able to control the device from an external keyboard. I liked some of the presets and those I didn't like I modified with my own creations. However, being limited to just the onboard keyboard was a pain for anything outside a hotel room.
So, when the version came out with a MIDI port (not the 5 pin DIN version, but with a handy adapter to connect to a 5 Pin DIN included) I thought it worth purchasing that. Indeed it was. I can now use the device on its own while away or hook it up to a controller when back in my home studio.
I gave the old JT4000 to my daughter so she could learn the basics of ADSL etc and kept the JT4000M for myself.
The JT4000 should have included the MIDI facility from version one, but the "M" version ensures that newer purchasers interested in this compact little beauty need not have to buy another one to use it in a studio set up without resorting to complicated methods to access it externally.
PS It would have been nice if Behringer had offered a discount for those of us who'd bought the JT4000, if we then upgraded to the JT4000M.