Not sure but maybe you misread the post?  It states that Windows is clearly able to accept more than 4 GPU's, but that it's currently unable to mine with more than 4 GPU's.  I know of many Windows based rigs out there with 4 GPU's and all 4 are mining at optimum speeds.

It's possible that it might be as OS issue, specifically a 32-bit OS issue.  Your 4 GPU's alone have 4GB of memory, which is the addressable memory limit of a 32-bit OS.