64-bit OS, CPU?
It depends on if you are running a 64-bit OS that can even use the 4GB you have.
My advice is:
If your cpu is 64-bit with hardware virtualization support - plan on installing Windows 7 64-bit when available and decide based on that future.
If your mobo and cpu support it and you are using or will be using a 64-bit OS, I say load it up!
Unfortunately, prices vary wildly for 1GB vs 2GB vs 4GB sticks and getting the right fit (using what you have or ditching it) is not so simple. I am not even going to start anything about DDR2 vs. DDR3
Assuming 64-bit, I think 8GB (4X2) is the magic number for desktops and 16 GB or 32 GB for Servers. Laptops generally aren't made for more than 4GB so far (sniff!)