If it is the wrong Mk4 (ie it is the navi-radio unit) then as you rightly suspect, there is no hardware inside the Mk4 to drive the Bordmonitor and from what I have discovered, the resident firmware that runs this unit is different (as you don't get maps as such, just direction arrows etc).

How was the unit advertised on ebay? Did it specify it was for the Radio Nav or did it say it worked with the widescreen colour monitor? Do you have the ebay item number?

Probably the best you can do is to sell it on and buy one specifically for the Widescreen....