As said, you can achieve this by simply running separate instances of the miner for every GPU.. You have 4 single-GPU cards, so you assign one instance of cgminer to each card, presto. Yes, those 3 extra processes are totally going to drive down your efficiency and clutter your system... Why don't you start by switching over to Linux, if you want to increase efficiency, since cgminer is known to have high CPU usage on Windows due to porting. Set up some scripts and you're good to go.