Are you using 64bit OS? 32bit OS can support 3.x GB of memory only.
Current 4Gig stick - Kingston KVR1333D3N9/4GB DDR3 CL9
Any recommendations for another 4Gb stick?

Current 4Gig stick - Kingston KVR1333D3N9/4GB DDR3 CL9
Any recommendations for another 4Gb stick?
Buy back the same specs, ie DDR3 1333 CL9 (and should be 1.5V), but usually preferably the same brand as well to minimize potential compatibility problems.