The point being he is in a competition class that limits to 4000W, so he can add up to that however he likes but not go over. IA 20.1 is rated at like 2800 so a pair of those is dead too, though the IA lineup is very good for SPL competition purposes.
Yeah
Looks about the size of my old one, but that new one is rated 4200W which is a deal breaker for you. Now I'm not even sure if the old version was rated 4000 or 4200??? We just called it 4K, but our org here limits at 5K so it was a moot point for me.
I don't think you want to wire them way below minimum impedance and count on "rise". I think a lot of these amps blow up that way. I can tell you that you might be impressed with what it will do if you wire it up to its rated nominal impedance. Search around for Taylor Fade's "7K roundup" and you'll see that the high voltage JBL/Crown 6K running 4 ohm made higher SPL than many of the Korean 7K amps at 1 ohm. High voltage is pretty slick and you may well rise from .5 ohm to 2 ohm anyway, you will have absolutely no idea. Now depending how serious you want to take having the amp(s) see .5 ohm or whatever, be prepared to try different coils until you get there with potentially a number of recones. Either way, the guys who are super comeptitive are spending a lot of money either blowing amps or subs, or tearing down subs to try different parts to optimize performance in their box on their amps.
I didn't think of Mmats amps though. That is a very good call. A little pricey but they are made in USA still (at least their higher end should be) and they make low impedance models even. I've never heard anything bad about them or their customer service. There's a youtube video of their 2K crushing the 2K rating on the amp dyno so a pair of those may well be your best option if you want to squeeze every watt you can in 4K rated class.