If you haven't read it, check out the Harman International white paper by Todd Welti, Subwoofers: Optimum Number and Placement here:
There's other good stuff there as well.
I've got four 18" JBL subs in my room, and I can place them at the midpoints of the walls, one of the optimal positions for multi subs. The notion of adding two additional subs in your room can have merit, though there are likely a few caveats.
1. It's preferable but not mandatory to have the subs matched. If they're matched, there's less futzing about because their characteristics will be similar and predictable.
2. If the subs can't be matched, then having two similar pairs is good enough. (see below)
3. Utilizing two different pairs creates some added complexity from the standpoint of different output characteristics and response curves. It's more work but not impossible to EQ this effectively.
4. Mixing four different subs in a room is a PITA, and it's good you're not looking at this.
5. Since midpoints of the walls seem out of the question, then corners is the next bet, as commsysman points out. Get those front subs as close to the corners as you can. Since you're in the building stage, it's better to do it now, even if it sets you back a few days to do it.
My four JBL subs consist of two S1S-EX units and two 4645C units. They all have the 2242-HPL 18" driver in them, so they pretty close to an exact match. Placed at the midpoints of the walls, there's not a dead spot or a boomy spot in the whole room. Due to the small size of my HT (15x17x8.5) my Synthesis® calibrator recommended having no door in the left rear corner; rather I left a heavily curtained opening so the excess LF waves could bass dump into the adjacent music/fitness room rather than kill us with concussive pressure. (OK, that's an overstatement.
Read Welti's paper, referenced above, and good luck!