Actually, I think the MT-32 was originally designed to expand the sounds available when hooked up to a keyboard (the musical instrument, not a computer keyboard). My understanding is it wasn't especially successfully sales-wise for this purpose, and it was Sierra that realized the potential for the MT-32 to be used for games, since there already existed MPU-401 cards that could be used to connect an external sound module to a PC (such cards presumably being meant for musicians, not gamers). Sierra then made a deal with Roland to market and sell MT-32s, and the MT-32 earned a new purpose in life as other companies also began supporting it.
Most professional synths that are used in recordings have either built-in keyboards or are rack mountable sound modules, which basically means they have metal on the side so that they can be screwed into a rack, like shelves. Since the MT-32 was just designed to sit on a keyboard, or later a computer desk, it hasn't held that much appeal for professional recording musicians. It is also seen as more of a casual device, I think, with less sophisticated tech than its rack-mountable brethren.
In particular, the MT-32 does have a synth keyboard cousin called the D-50, which was a very popular device with musicians as I understand it, and although I don't know anything about Rocky IV or its soundtrack, it is possible that this, or something similar, is what you are hearing. The rack mountable version of the D-50 was the D-550.
The D-50 isn't 100% MT-32 compatible, so it can't be used for games the way the LAPC-I, CM-32L, CM-500, or MT-100 can. But it does use the Linear Arithmetic technology that the MT-32 employs, so they would have similar sound characteristics.
Hope that helps! Someone else here may be able to peg the precise device used in the Rocky IV soundtrack, or correct any inaccuracies in what I've written.EDIT:
Here's a forum post where one of our members, Cloudschatze, shares a scanned ad where you can see how Roland first marketed the MT-32:http://queststudios.com/smf/index.php/topic,2633.msg28347.html#msg28347
I forgot that the emphasis was more on digital pianos than keyboards, but that does make sense, because digital pianos at the time would have had more space on which to set the MT-32, few if any alternate sounds of their own, and would benefit from the MT-32s accompanying instruments. Hence the "miniature orchestra" tag, and the default channels of 2-10 as Cloudschatze points out, leaving channel 1 free for the piano.
I also have a Roland User Group magazine lying around somewhere. Basically a catalog advertising this era of devices: D-50, MT-32, PR-100, etc. Though as I recall the info in it is extremely basic, just specs and things, so I don't imagine anything useful can be gleaned from it.