If they are parallel then in theory it's just a single subwoofer. So corrective filters for one sub position only. So now if you want 4 subs to run in parallel, which sub position are you going to correct while leaving 3 other subs running the same correction in 3 different positions.
I'd suggest, just have dual subs setup, each one close to your front channels.
However if you really want to get down to it, then yes a minidsp is the way to go.