First, verify that you have what you think you have.
Does cpu-z show 4096 Mbytes?
Right click on computer/properties.
Under windows experience index, does it show 4094 mb?.
does it show 64-bit Operating system?
Mine does. Could you have loaded a 32-bit OS, Or could some RAM not be seated properly?
It is unlikely that your games will try to use 4gb of ram. Since games are built to run an many configurations, I doubt that a game would be coded to detect the presence of a 64-bit OS, and more than 4gb of real memory, in order to take advantage of it. By default, a 32-bit application addresses 2gb for it's own private data, and shares 2gb for the OS. Some games can be patched to try to use 3gb of private area. Backing up this addressing space is real ram, which can be used for everything. For a single application or game, it would be unusual to need more than 3gb, so you should be ok. The value of 4gb or more is that more than one application can reside in real ram at the same time without displacing each other and causing demand paging I/O.