I've read different things online about this as I too toyed with mixing ported with sealed subs.
I think that the big "gotcha" is that it is harder, a lot harder, to tune subs to the room to begin with. I am not talking about the subwoofer crawl because that pretty much goes away if you can get each of the 4 subs in the middle of each of the 4 walls.
I am talking about your receiver or processor. It will have to adjust itself to the combined output of each sub, and with different capabilities, you may have 1 or 2 subs playing one frequency great, and the others getting boomy.
I just went through a room correction setup about 2 months ago where I had to adjust 1 sub at a time until I got it dialed in as best as possible, and then shut it off and went to the next sub, and then turned them both on and went at it again.
Luckily for me, the subs performed pretty much identically (one SVS and the other a DIY virtual "clone" of the SVS), but it still took a solid 90 minutes of tinkering around. Then again, my 2 subs are both up front by the main speakers, so I would notice distinct variations just by moving a sub just a couple of inches sometimes, or if I tinkered with the phase a little.
Again, I think that it can be done, but you might actually be sacrificing the capability of one type/model so that you aren't "over-pushing" the other. I know that it seems like "hey, I will get the best of both worlds" which is true, but you will also get "the worst of both worlds" if you want to keep the LFE and bass clean and not boomy.
