The scandoubler in the 3000 is a great advantage!
The 4000 with the mediator lacks the videoslot so an internal scandoubler is not possible.
This means a external one with a switch.
While in the 3000 there is the scandoubler build in and saves you the high! cost of a second hand scandoubler.
Maybe it is wiser to buy the tower for the 3000 for the money of the scandoubler.
This hack for the 3000 is something of a wire soldering I believe.
Not to hard to do I believe.