it isn't as black and white as that.....some machines will recognise the full 4GB, some won't. Depends on the motherboard BIOS and the northbridge involved (but please don't ask me to give examples.....)
Last year we installed a bunch of Dell Optiplexes which DID appear to see all 4GB. At least they reported all 4GB. Whether they actually used it is another question....
But the reality is (as has already been said) on a machine of that age and performance, there is likely to be little visible difference in performance between 3GB and 4GB