When you say the memory drops to 3.99GB, what exactly are you refering to (Installed Physical Memory, Available Physical Memory, etc)? If you're looking at Avaialble, ignore it, it's a useless number that has wasted so many peoples time trying to troubleshot something that isn't broken. Even if you're trying to troubleshoot a low memory condition you wouldn't use that stat.
I'll leave what I wrote above but I just noticed your subject was "usable RAM". If you're running Windows 7 in 32 bit mode, you won't be able to use all 6GB of RAM. If you right-click on My Computer and select Properties under "System type:" do you see "64-bit Operating System"? If you don't, you'll be limited to 4GB of useable even if you have 6GB installed.
’06 K1200R Corbin w/ smuggler, Wilbers shocks, PCIII, Factory Pro Dyno tuned, CC Suspension plates, Vario levers, Dual Evo filters, Leo Vince SBK, V1, Zumo 550
’02 WR250F Stock
’99 SV650 GSX-R front end, Aftershocks suspension, Penske shock, 41mm FCR flat slides, Bridgestone slicks, M4 exhaust, Vortex clipons, Woodcraft rearsets, Hot Bodies race bodywork, Factory Pro clutch cover, Total Loss ignition, BRG relocated battery, APE Adjustable Cam sprockets, JE 13:1 pistons, mild porting, balanced crank, Sunoco 110 race fuel
’98 KLX331 Race Tech suspension, 331 Stroker big bore kit, 33mm pumper, Tag bars, Stroker exhaust, IMS desert tank, tall seat foam w/ Stroker cover, Open air box, KX clutch