The Hornby website suggests this unit was withdrawn as it did not perform very well and that it used "feedback control" [not sure what that is] as opposed to DCC [Direct Command Control with an integrated signal in the power line] which has much more flexibility in use and is compatible with all the modern stock. It's a bit beyond me as my Hornby stuff just had the basic variable resistor
. but I can see the attraction. I'm sure the 4000 must be more sophisticated than that if you get it going [and the train].!
Dave W