If you have four subs, or for that matter 2 subs and 2 vents, the general rule of thumb is never put them more than 90 degrees away from each other in a vehicle. I have seen SPL vehicles that do not, but the time and effort required to have them work together without cancellation is large and most people doing a daily driver vehicle would rather not build 12 boxes in a row for the same sub(s) trying to get it right......

The purpose of the phase control in your deck or on the amp itself is to help get all of the sound from the speakers to your ears arriving at the same time to avoid cancelling each other out. Since the path-length for your front speakers is so short compared to your subs, for certain frequency ranges it can help to give a more coherent sound if the phase of the sub is reversed by 180degrees. If the front speaker cone is moving out towards you at the same time as the sub is, not only is there a time delay for the sound to reach you due to the distance it has to travel but at the same time, if the peaks in output from each point are out of phase the overlapping frequencies will not be as loud or as clear as they should be.......reversing the phase of the sub is an easy way to try and clear that up......if you want to dial in your system for best possible sound from the driver's seat and screw the passengers, time alignment and phase should be experimented with right from the tweeters and mid-ranges back.......it helps to have a buddy to help with this........

Home audio subs have bottom or rear-firing vents quite often, but even they need to be set-up properly ie. corner or wall loaded so the vent energy is in phase with the speaker ouput from the front of the enclosure.

HTH