Well the user guide is right as I looked at the manufacturers website about that board and it said 2gb max also. So get a new board if you want higher than 2 gb of ram. However, the requirements for this game is:
OS: Windows XP 32-bit or better.
Processor (CPU): Intel Core i3 mobile, Intel Core2Quad or AMD FX 4100+
RAM: 2 GB.
GPU: Nvidia GeForce GT8800 , Intel HD 4000 or AMD 4650+
Sound card: compatible with DirectX 9.0c.
Free HDD space: 30 GB.
Internet connection: 1024 Kb/s or better.
So I would not be worried about ram but rather your processor and possibly the gpu. Your processors is good except the quad part so that means you have 2 cores when you need a 4 core processor. Also what is your gpu?