Gateway service has confirmed my suspicion, echoed by a couple of people
who responded to the original request for help.  The ATI VLB video board
uses the addresses for COM 4.  They could suggest no work around.
I will be returning the DF IO card they supplied for COM 4 (even though it
could not possibly work) for credit against a bus mouse.  This will free
up the COM port I need - too bad the original salesman who suggested either
the DF IO card or the bus mouse would solve my need for a port didn't know
enough about the hardware.

Otherwise I must say that the 486DX2/66 system has worked very well - no
problems with any other hardware or software.