If you've already got the ram and it works in three slots, then I would leave it alone. The extra ram will run faster even if it's not in dual channel mode. Your board manual will tell you which 3 slots are the best to use.
No. not at all. It either works or doesn't. Don't think you have to rma the board; leave it alone or remove one of the sticks. You can also add another single 2 gb stick if you can match it to the other three.