It is slightly better to use two modules than to use four because it puts less strain on the memory controller. That also leaves room for more RAM if you upgrade the RAM again before you get a new machine.
Thanks for the advice. Ended up going with two modules (more expensive, but not enough to break the bank). Unfortunately my motherboard is old enough that 8GB is the max memory. So next RAM upgrade comes in the form of a new rig when this one dies.