Your 4-way speaker sounds like a great adventure. I have been in the idea stage of building a HE system for almost a year now.
I have spent allot of time reading and tail chasing, but nothing built yet.
Being a newbie, one has to pay their dues.

I have thought about a four-way system myself, but size, cost, and complexity seem to be in my way.

It might help if others knew what midrange driver you intend on using, to include loading type, horn, BR, or sealed for example.
And possibly include your listenig room volume to help with your needs.

My thought is if you need the output of four 12" midbass drivers from 60Hz to 250Hz. Possibly two 15" drivers would work as easily while being a little more managable in a verticle stacked position.
A driver like the JBL 2226J for instance could be loaded in a mangable box, BR or sealed for allot of SPL's out.

Here is a design that you may, or may not be aware of in the following link.

Hopefully it will give you some ideas.

Keep us posted of your progress?

Norris

• http://www.audioheritage.org/html/perspectives/drews-clues/system.htm