Yes you should be able to match the speed, and preferably the brand of memory currently installed.(including cas) Some boards are more picky than others.
Best would be to buy all new,to prevent possible conflicts, and sell your current RAM.

Also depending on your Operating System and motherboard, you may not be able to acess 4GB of RAM. You can install 4GB but your system will not be able to access it unless you are using a 64 bit OS and the motherboard supports that amount.

Here's a good explanation if you need.
4GB RAM in 32-bit Windows XP and Vista