dimensions

Seeing as you're going to so much trouble with your system, it seems a shame not to do it the right way and put each sub in its own individual compartment, with its own port.  

As for your amps, if you are running 4,000 watts (or 8,000. We think you meant "two 4k 2ch amps") You will utterly destroy your electrical Just to push 4000 watts SAFELY in your vehicle will require a 310+ amp alternator, 8000 watts will need a MINIMUM of 600 amps on an alternator setup...

But then again if the 4000 watts is what is stated on the amp, then we are 99.99% sure that your amp is not CEA compliant. To figure out how many watts your amp really has, measure your car's volts at the battery at idle, then multiply that by the fuse on the amp(s). That is what you are really going to get from it. We know of some amps that state they operate at 8000 watts, but even the fine print doesn't tell you that the 8000 watts come from a 24 volt (or was it 36volts) system, which would mean your vehicle is a brand-new Advanced Semi-Truck or a Cruiser yacht.

To push the subs at RMS, your alternator will need to be a 275 amp alternator setup. Adding batteries will give you a slight buffer, but only for mere seconds at most. For example, if you have say a 1400 CCA battery, you'll get about a 2-3 second burp out of your speakers before they deep cycle the battery and potentially damage the alternator and other electronics (brownouting them)

As for an answer to your question. Assuming you are using a 7 cubic foot adjusted enclosure, your rectangular port will be 10 inches by 3 inches by 11.5 inches deep for Street Bass. Now this is if the subs are in separate enclosures. Four 15s will need 28 cubic feet of space if you want them. These are designed NOT to be placed in shared chamber setups unfortunately.

If you are looking for SPL Bass: 5 Cubic Feet enclosure, same port but 1.5 inches deep. This is per sub as well.

So without knowing what you are looking to do for your setup specifically, these numbers are only general guidelines.

Good Luck!