Windows 8.1 uses only 50% of my installed 4GB memory which causes games to crash. System information state that 3.71GB are usable). The picture below is from my Task Manager showing the memory (Arbeitsspeicher in German). The graph below shows the memory usage for the last 60 seconds, and to the right you can see that roughly 50% (of 3.71GB) is used. 60 seconds before I took the screenshot, a game crashed due to memory problems.

I also noticed that memory composition (the graph below) remains fairly stable when it comes to "Free memory" (far right). Mouseover information says that free memory can be used once necessary, but it doesn't seem to happen. Memtest86+ says all memory is working fine.
System specifications:
- 64bit architecture
- dual boot with Ubuntu 14.04
- pagefile.sys set to 6144MB manually (as an unsuccesful attempt to solve the problem)
This question is not related to two other questions (How to enable 4GB in my Windows 7 64 bit?) and (64-bit Windows 7 can only use 3.25GB of RAM instead of 4GB?).