"On boot up I see cpu is running 200mhz. Should I increase this to 400mhz in Bios??"
You haven't said which operating system you're using, but if it's a 32-bit OS, then it will only be able to use around 3 - 3.2 GB of memory. Will it boot okay with three modules installed? Although you should be able to (normally) install the four modules, even if it doesn't utilise it all.