I honestly think that this is just a limitation of the GUIminer. My best opinion would be to switch to a miner that has known support for 4 cards.
Since it seems as if you want to stay away from the CLI, I can script it out for you quite easily. Just name a miner of your choice.
I have seen screen shots of 1, 2, 3, 4 different GPUs on GUIMiner, however, if they were individual cards or 2 Multi cards i'm not sure.