If you need a fourth spot, the frame bosses are there. If a standard bottle is not to your liking, you could use an aero bottle as well. If three standard bottles + one aero bottle does not satisfy your fluid carriage needs, then you are outside what we consider ordinary athletic considerations for a long course athlete, particularly on a supported ride like an Ironman race. Of course, if you insist on five round bottles, then Omni may not be for you.
Storage is meant primarily to be done with the integrated. You could also mount a small saddle bag between your saddle and the Beta carrier, by adjusting Beta rearward. And if that were still not enough, you could use the seatpost bosses for additional storage options.
That is our recommended setup. But there are lots of variants an athlete could choose, given the many boss locations on Omni.