The amount of usable memory are not the exact amounts. Usable memory is a calculated amount of the total physical memory minus "hardware reserved" memory.The reduction in available system memory depends on the configuration the devices that are installed in the computer and the memory that is reserved by those devices
You can try updating the BIOS on the system to check if the usable memory changes. Please enter your service tag # on the link below, select the OS, then download the BIOS from BIOS section onto the system and install it.
Do you have a Dimension 9200, or as the heading states, an Inspiron 9200? The Inspiron 9200 is a laptop, and its chipset does not support memory mapping above the 4 GB boundary. Even if you are using a 64 bit OS, this condition must be met in order to use all 4 GB.
A Dimension 9200 is a desktop that will use all 4 GB provided a 64 bit OS is used.