Short answer is you can't. It's dependent on the Intel Architecture. It depends on chipset and some other factors, but basically 32-bit OS have a limitation within the 4GB boundary. So Intel didn't anticipate people would use that much memory in the original x86, so that's why that limitation exists. The missing memory is tied up in Video memory and some other things which must be below the 4GB limit.
If you are using a computer that has over 4 gigabytes (GB) of memory installed, System properties, Microsoft System Diagnostics (WinMSD) or other system utilities report a memory value that is 256 megabytes (MB) less than the total physical memory that is installed.
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This issue can occur if a server is using the Intel Profusion chipset. In the read-only (ROM) memory of the computer, the upper 256-MB memory region is reserved for memory-mapped input/output (I/O) devices. The amount of reserved physical memory may increase depending on the number of I/O devices that are installed on the computer. Typically, a computer that has 4 gigabytes (GB) of actual physical memory looks as if it has 3.84 GB of total physical memory.
The Intel Profusion is an eight-way symmetric multiple-processing chipset that is designed for enterprise-level server programs. It focuses on raw processing power and high-throughput I/O performance that is commonly found in 8-way Dell, Compaq, Hewlett-Packard (HP), and IBM servers. The amount of memory that is consumed can be larger than 256 MB; however, memory increases in 256-MB increments.
" The MacBook Pro Core 2 Duo presumably uses Intel's 945PM chipset, which can physically handle 4 GB of DDR2 RAM. However, a number of items that must be stored in physical RAM space, and when RAM reaches 4 GB, there is some overlap. In other words, in a 3 GB RAM configuration, there is no overlap with the memory ranges required for certain system functions. Between 3 GB and 4 GB, however, system memory attempts to occupy space that is already assigned to these functions. For instance, the PCI Express RAM allocation occurs at somewhere around 3.5 GB of RAM and requires 256 MB of RAM. Thus, the virtual space between 3.5 GB of RAM and 3.75 GB of RAM is occupied by PCI Express data. So in a system with 3 GB of RAM, nothing is being wasted because the memory space required by PCI Express is still between 3.5 and 3.75 GB, and the installed system RAM does not violate this space. The net result is that at least 3 GB of RAM should be fully accessible, while when 4 GB of RAM installed, ~700 MB of of the RAM is overlapping critical system functions, making it non-addressable by the system."