Hi Andreas,
In properties does it say (something like) Installed Ram 4GB (available 2227mb)??
It might be a setting in your bios, I had a similar issue when upgrading to 4gb from 2gb, though mine said 3.25gb installed and xxx available.
In the bios you need to look for a setting called something like Memory Remapping / Memory Remap. In my bios its under Advanced >> Chipset configuration. Make sure that setting is enabled.
After doing this my pc automagically told me I had 4gb installed.
Remember also that your machine will use roughly 25 - 30% just to run. which, if added to your 2227mb (just over 2gb by the way) would bring it up to the 3.25gb or there about.
Of course, if you...