Didn't see this post as it's buried in the wrong forum. Your resident mk expert comes to the rescue.

First regarding your question, which other people seem to have ignored.
For the speakers simply mount a y adaptor at the actual speaker cables. Then you can plug both the pc and board in at once. You will need an amp for the pc end of things though, as I believe mk4 boards had built in amps on the main board, like the rest of the mk series.
No you will NOT be able to simply plug it in, or use a multi-jamma adaptor to use both at once. Mk4 has more than 4 buttons, and thus it's not fully jamma. It has an additional kick harness for the rest of the buttons. You'll need to purchase an additional kick harness and hack it to the jpac (not much to hack, but you want to keep it original as i understand it) and perhaps make some sort of custom switcher between the two.
You aren't going to like any mk game as a dedicated cab, simply becuase it doesn't have the standard 2 rows of 3 buttons sf layout. It makes it quite akward for other games. DO NOT hack it though and here's why.
Contrary to popular belief the mk4 cab will become exteremely popular in years to come, for several reasons:
1. Although nothing will ever top mk2, mk4 almost held it's own in that dept. Considering the dwindleing interest in old school fighters when it was released, it was extremely successful.
2. Mk4 has the unfortunate distinguishment of being one of the last midway arcade games ever. (not counting racing games, which use more of a gimmick machine)
3. It's also the last of the games to use the classic mk2 cab design.
4. It's the last mk game that will ever be released in the arcades, according to midway.
5. Because all the mk games used exactly the same cabinet, finding one that is original to the game is extremely rare. Most mk1 cabs are upgraded to mk2 and mk2 cabs up to mk3. The beauty of mk4 cabs is generally the original side art is in tact, which is the most valuable part as midway flat out sells a different sideart if you are going for a kit form.