You can utilize all 4 GB running programs on a 64 bit OS. That's why I went with 6 GB for my new PC, it will be hard to max out those extra two.
If the motherboard supports triple-channel, then yes, 6GB would work out better, however, if my memory serves me correctly, LGA1156 motherboards typically feature 4 DIMMs not 6, and triple-channel isn't supported. Current memory prices are really inflated right now and there is no sensible reason to be "generous" when purchasing it.