I've closed all non-essential programs that I could, but I'm still using 5.5 gb of my 8 gb of memory. I've added together the working bytes displayed for each process in ProcExp and it adds up to a more reasonable 1.5 GB. Where is the other 4gb of memory usage going?
you are always using most of your ram. aside from the 1.5gb used by your processes, the rest has cached data that is not in use, but awaiting use. This is why ram helps speed up computers. it store data that it predicts you will need so you avoid fetching them from the HDD.
That app you used does not properly understand this concept, and reports you are "using" 5.5gb while in fact you are using 1.5gb and 4gb is cache. Try using task manager, it is much more accurate
Are you talking specifically about memory amount listed as "In Use" or are you including the amount listed as "Standby" as well? The "Standby" memory includes things like OS disk cache that won't show up in normal processes since it is a file system driver function.
Memory usage ("In Use") is split into multiple pools and the "Private Working Set" is only one of them. The Commit Size should give you a better indication of how much memory a process is potentially hogging since it counts all memory that has been requested by a process instead of only specific subsets.