eww, they look like 6.5" mids
only take about 800 watts total, I would strongly suggest you save another couple weeks and get what you want the first time, everyone does it they want stuff now, but in the end you will want better subs and will lose money in doing so.
my suggestion keep with the 3 subs, save another $100-150 and get RE SE's Fi SSd, Dc level 3 ect.
but thats just me. GL with your choice.
IF you do chose to get 4 small subs, DC level 1's will handle a lot more than those RE REX's and they are about the same price, and will look a lot nicer.