"Fresh install of Windows 8 Pro on a vertex 4"
I don't know what to tell you then. Windows should certainly be caching a lot more. Looking around at all four of my Windows 8 machines, my 32GB box is at 17GB cached with 8GB free. My 6GB box is at 0MB free with 4GB cached. My girl's 8GB machine is at 2GB free with 4GB cached, but that Windows install is only a couple days old. All three of those are running SSDs for the OS drive with HDDs for data drives.
Booting up my 8GB Mac Mini, it capped out at 2.0GB cached within 15 minutes. That machine has literally had Windows 8 installed on it, then never been booted back out of OSX since Windows' installation. Your machine that is used for many things often is acting like my "never used" Mac Mini installation. Something is very wrong, but I do not know what to tell you outside of the troubleshooting that I pointed you to earlier.
Now that I've run around the house looking at Task Manager on all these machines, I've noticed that my main machine is reporting a super-low memory clock. I had always just assumed Windows 8 was using the bus-clock instead of the labeled effective-clock, but everything else is reporting memory speed that's on the label. Off to do some troubleshooting myself!
EDIT: Mac is at 2.7GB after 30 minutes.
EDIT 2: I even pulled my 3GB Windows 7 laptop out of the closet. That thing even precached 1.7GB before running out of ram to use.Edited by darklordjames - 3/17/13 at 1:02am