If you have a Lenovo G550 2958 with the GL40 chipset, then you already have a 64 bit Pentium T4200 / 2 GHz processor.
https

/www.cnet.com/products/lenovo-g550-2958-15-6-p-t4200-vista-home-premium-64-bit-4-gb-ram-320-gb-hdd-series/specs/
The problem is most likely NOT your CPU, but your version of Windows. You most likely have a 32 bit version of Windows installed and you will NEED a 64 bit version of Windows in order for you to install more than 4gb and have it recognized by the operating system. 32 bit versions of windows cannot recognize more than 4gb of memory. Based on that information it looks like your system CAME with 64 bit Windows Vista, but somebody may have changed it to a 32 bit version of windows since then.
It may also...