Yes I know that. How bout if I ran both speakers parallel. That you be 2+2+4+4=12 12÷2=6 then 6÷4=1.5 ohms, if the subs can withstand a 1 ohm load , how come I cant run a 1.5 ohm load?
It's doable, but they subs won't get the same amount of power. If your amp is stable down to 1 ohm, then it doesn't care how you get there; it only cares about the load it sees at the speaker terminals.
Having said that, I don't know where you came up with that formula for calculating parallel loads. If all parallel branches are the same resistance, you can use the formula Rt = Rx / Rn ; Total resistance equals the value of each branch divided by the number of parallel branches. If each branch is a different resistance, then you use the formula Rt = 1/(1/R1 + 1/R2 + 1/R3 ....).
In your case you could technically use either formula, but let's go with the latter:
Rt = 1/(1/4 + 1/4 + 1/2 + 1/2);
Rt = 1/(6/4);
Rt = 2/3 ohm, or roughly 0.67 ohm, which is lower than your amp can handle.
Your best bet would be to wire the dual 2-ohm sub in series to create a 4-ohm load, then wire all three 4-ohm loads in parallel. Thereby, Rt = 4/3 = 1.33 ohms.
As stated by others, having two different subs or sub with different impedances will not give you ideal results, acoustically speaking, but if you wire them correctly, it is far from impossible.
Bear in mind, I'm just some guy on the internet, as is everyone else who provides advice in forums. The information I gave you is electrically correct, but follow it at your own risk. If you didn't know how to calculate total resistance, you might be better off either keeping it simple (get two of the same subs) or paying someone to do it for you.
On a side note: I don't know how running two subs in phase with eachother would create any cancellation. I would love to hear the theory behind it.
Hope that helps.
- Joe