If you have a Lenovo G550 2958 with the GL40 chipset, then you already have a 64 bit Pentium T4200 / 2 GHz processor.
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 be dependent on your laptop motherboard. The amount of RAM that is supported is often predicated on what the motherboard manufacturer intended for it to support. Even with a 64bit CPU and 64bit windows, if the motherboard itself does not support more than a certain amount, or a specific SIZE of module, then you cannot do it.
There is actually more to the model number than what you have given though, and could be significant differences between sub models. Knowing the EXACT FULL model number would be helpful. It should be printed on a sticker on the back of the laptop OR under the battery in the battery compartment. Examples would be 2958-A5U or 2958-A4U among others.