Philip,
Have you run taskmanager to seee what type of process are taking up the memory? You may also want to download " procexp.exe " from Micorosft site to better understand the process details.
Also depending on ## of users the server support 28Gigs sounds way too for the BI4 Server, I would very strongly recommend min 48Gigs. Double-check the sizing guide for better guidance
Sizing and Deploying SAP BI 4 and SAP Lumira
Regards,
Ajay