Wait, you starts with ring radiator and 4 woofers, now you are doing a horn and two woofers???
The concern you have with multiple woofers like your original design is interference and comb filtering. The PLD of the top woofer is different than the bottom woofer at any listening position. They will not act as a linear array above a frequency that correlates to the spacing. That's why successful line arrays use many tiny drivers, or heavy processing to align them (in pro audio).
Paradigm Has similar designs, except only one woofer is the midrange. The rest are midbass.
If I had that budget, I would buy Paradigm Reference or Dynaudio.
If I was building your enclosure and crossover, it would be a 3-way design and the top woofer would be in its own enclosure volume.