I only see the information about Windows Vista 32bit. How to see that the x61s is a 64bit machine?
Your Lenovo link indicates that your X61s uses the "Intel® Core 2 Duo L7500 processor." From Wikipedia, "'Merom' (low-voltage, 65 nm) All models support: MMX, SSE, SSE2, SSE3, SSSE3, Enhanced Intel SpeedStep Technology (EIST), Intel 64,..." The chart indicates that your L7500 is a Merom class processor.
The Intel64 link explains, "x86-64 is a superset of the x86 instruction set architecture. x86-64 processors can run existing 32-bit or 16-bit x86 programs at full speed, but also support new programs written with a 64-bit address space and other additional capabilities." [my italics]
Even though your X61s, as mine, came only with Vista 32-bit you can upgrade the operating system to Vista 64-bit. This would allow you to use all 4GB of RAM.