Not sure if Pijush has mistyped or what to be honest.
If you want to have more than 4Gb useable RAM installed you must have a 64bit operating system. Personally, I have had absolutely no problems running a 64bit system and it performs better in just about every task (again from my experience) with more RAM it can use.
Which version of Windows 7 are you planning on? Different versions of Windows 7 64bit support different amounts of RAM also, see here:
However, the operating system is not necessarily the only limiting factor. There can be hardware limitations that mean it could only support up to 4Gb RAM even with a 64bit OS. Which exact Lenovo G40 is it as there may be some hardware limitations.
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