split it up and fit two gig in each channel
Most manufacturers colour code the memory slots into channels - so you may have two blue and two yellow (and not necessarily side by side).
The idea behind the above is so you can put matching pairs in the same channel - this will allow the memory controller to perform "wide" memory operations and write/read from the two memory modules at the same time.
Putting the memory modules in different channels will result in the memory controller having to perform two operations rather than the one. In real life this is hardly noticeable unless you are really hammering the machine.