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.
Oh my god.
It is really stupid regardless of whether you are on Linux or Windows to have to run multiple instances. Especially since Cgminer is already set up to use all GPU's with one worker and display stats on one screen. Why would you want to have multiple screens that you have to toggle just to get stats.
Windows CPU usage can be reduced using affinity so I'm not worried about that.
Switching to Linux at this point would take quite a bit of time for all of my computers and still does not solve the problem. A lot of people like to run on Windows (Bloated that it is) rather than Linux because it is far easier.
Given how Cgminer seems to work now, I would think (but I do not know) it would be easy to add another cmd line option to use different login names per GPU.