I reccomend booking the 4 together each way. Easier to keep track of 2 numbers than 8, plus when you check in you will get the next 4 numbers, if you do each seperately, then someone could get a "B".
I didn't do that for a recent reservation, and of course the day after the sale ended, a ding! was $10 per person cheaper, but only for one direction. It would of ended up costing $30 more per person to rebook, but I had done the one-ways would of saved almost one park admission.
If you cannot get 4 tickets at the lower price, Call Southwest, they can split the exiting reservation and into multiple reservations. This works by passenger, you cannot split the itinery.
I've done this in the past.
Tony