Since Vin7 (possibly Vista) Windows eats almost all available memory for the cache.
But this does not mean that it is not enough when you need to the Windows share, if anything.
What you have hardware reserved 4GB? That's a lot, IMHO.
About hardware redundancy read here
General advice is to update BIOS, play with settings of BIOS, update the driver - to take fresh from the manufacturer's website iron.