4 slots for only 2GB? I assume it's DDR1... If that's the case, 2 x 1GB is your best bet. Reason is, I don't think there are any 2GB DDR1 RAM module. Filling up all 4 will give you trouble should anything happens in the future.
But since it's and old platform, the safest is to have only 1 stick of 1GB RAM. Your system isn't that demanding nor have the capability to meet demands either. Plus, the price for a DDR1 module is just crazy!