Teggy as Nomaan already said, these are dual-channel memory slots. If you try moving BOTH modules to black slot instead of blue they will still work, but you can not use one slot from each pair.
HAving said that, for upgrade you also need a pair. This new pair will go in in unused slots (either black or blue). Make sure that you use identical RAM modules for each pair of slots.
PS: 32-bit operating systems use only 3GB of RAM (although they do detect 4GB).