I would think
No problem in making 1200's that will accept 4000 parts, or 600's for
that matter. Except for the fact that it would make it more
expensive for the factory to produce 600's and 1200's. The idea was
to make them cheaply as entry level machines.
However, you have a bunch of people who buy a 1200 and want it to be
a 4000.
There is also some incentive to make the buyer of entyr level
machines find it difficult to upgrade. It helps keep the sales of
the higher priced machines going. After all… C= didn't make any
money off the Warp boards.