Sounds like a dead short to me if you're saying it blows immediately when turned on.

Have you tried having 3,4 and just either 1 or 2 alone? I would add 1 display at a time and see if any particular display blows when connected. Start with one and go from there.

Okay now take one of the displays you know works and put it in each display spot one at a time 1,2,3,4 and see if it lights in all four positions. This will help eliminate the connectors to some degree as the problem. Look at the displays that don't work, notice any burn, check to see if solder is bridging any components, are they gassed out.

The displays are not that difficult to get. Place a wanted ad here on the site and of coarse ebay is full of the them. Cost you around $38-$40.00 at most per/display.

Did you take a display that worked and check each position out??? If you move only one that does not light you will never be sure the connector is solid. If you did this sorry I sometimes over look things.

Can you get to your diagnostics? If you can it would also help if you could put the two displays that light in 1,2 first and roll through the test. Your looking to see if any of the digits come up missing segments when it goes through the digit test; test both 1,2 and then 3,4. This will help rule out a bad chip on the MPU which if bad could cause the display to go out.

I'm not sure so I don't want to give you bad info. You should visit the pinwiki site and read up on what you should check.