I say try it... 3 or 4... whatever fits. But bear in mind a pair of high powered 15's in a 4th usually takes up space to do it right. If someone says its possible than it probably is. Honestly I haven't crunched numbers yet as Bobbys probably close. I just like to troll his ass. He gets so spectacularly defensive its Snowflakeville and Im bored. Comon' Mr Fisherman... you had to suspect. I think you would get close on a 4th but you'll never know till you try. I heard all 4 at 1st so try that 1st then scale down till it works if needed. Watching airspace and port.
Even 2 higher powered 15s in a 4th can get pretty big if done right. Its all ratio & goals then materials, displacements etc. And some math... ez. Keep the tuning in the 40s fwiw.
Ratio, goals, subs and power... ez. Don't forget install/config too.
And make sure the subs have an EBP usable in a 4th and compatible.
Every box is a collection of compromises so don't feel like whatever wont work. Maybe post a attempt on paper or screenshot and give it a try. Only way to learn unless you wanna burn up some lumber (and hrs).
I think Bobby will get it close, I just want to see him try.