I think it's a chipset limitation but I'm quite sure that motherboard won't support more than 4GB RAM. You best option might be to install the single 4GB DIMM.
Thanks for your answer. A few more questions
1. What will run faster 2X2GB or 1X4GB?...
2. One of my RAM stick(2GB) still works. DDR2 RAM is really expensive in India. I was thinking of buying another 2GB stick.
3. Since MB is limited to 4GB what is the advantage and point of having 1 slot empty?...
1) If we assume the speeds and timings are similar, 2x2GB is better than 1x4GB since it'll run in dual channel mode.
2) Most DDR2 memory nowadays are expensive. I don't think it's worth to get another 2GB module.
3) The disadvantage is that you won't be running in dual channel but it'll still be fine. If you don't like to spend more, just install the single 4GB module and see if it works (I mean, it should but who knows).