I was in a similar situation to you not long ago, accept I didn't already have amps in hand.
I knew that I was planning to get a pro audio amp so my choices were slightly different. I had a few more choices about what load to present to the amp. I ended up getting four 1260's in 4ohm and running two per cabinet. So the amp sees 2ohm stereo for about 750 per enclosure. My enclosures are also sealed, not ported.
Have you done any modeling to see how much power the subs will take before they hit the limits of excursion? With 500 watts in a ported enclosure you may be getting pretty close, though I haven't run any numbers to be sure. If there is even a thought of purchasing 4 subs and that you'll be using the other two in the future you really need to think about how you plan to run them in the future before making your purchase. Do you plan to use the same amps on your future build? Do you plan to go sealed or ported on your future build? Would your amps support the load presented in your future build (if you plan to use the same amps...)?
Personally, I would say figure all that future stuff out now and just build what you plan to do in the future. I know it's not my money being spent on this, but these subs are super cheap and lumber isn't that costly either. Lumber (and other building supplies) and four subs you're only looking at like $350-400. I have no idea what another O Audio amp would cost you but I assume you could do this entire build for less than 8 or 900. And when the "future" comes around, you'll already be done and enjoying your subs. Use that "future upgrade money" for a really nice veneer or something down the road. Or buy the subs and build both enclosures and then just buy another amp when money is available. That way all you'll have to do is mount the amp in your second enclosure and you'll be done.
Sorry for just adding more questions, but you really need to think about future needs if you're planning to buy four of these bad boys.