I am looking to run 4 12" BL subs in my SUV and was wondering what would be the best tuning and internal volume for an enclosure that size? Also would it be better with subs up and ports firing back? I will be running 2 Rockford Fosgate 2500-1bdcp amps on this set up.

Tuning will be based on your personal preference in music and how you want it to sound.. Iv heard multiple people say they like the sound of higher tuning, and many people who love the lows so they tune low, so its hard to tell you what tuning to design your box at.

I actually want it to hit low. I am tuned right now with 2 12' s at 33hz and its loud.

Well.. If you like the way the setup is right now, Id go with about 2 cubes per sub.. So 8 cuft, keep the tuning the same if your enjoying it and yes, typically subwoofers firing upward, port rear is the best in an SUV type setup.