Faital Pro 4FE32 should be small enough for pillars and easily the loudest thing you can get out of a 4inch driver. 91db/1w/1m and it does come in 4ohms and the power handling should be very underrated as it's AES rated. That's alot harder than what car audio companies do. If you have clean power and a steep enough crossover I bet you could put a 100 watt amp on those for demo's and they'd do fine. (200 watts if it's an 8ohmer) Sensativity matches the sundownz pretty closely too since they'd be alot closer to you. Rated up to 20khz, virtually 0 inductance, but a tweeter wouldn't be a bad idea. If you really want to get loud with 4's you may want to run a pair, do a MTM combo in the pillars. It'd bring the impedence of the amp down to 4 or 2 ohms, not hurt SQ much at low volumes and give you an extra 6db of headroom for sure. Just angle them so they are at the same angle despite the height difference and the acoustic center will be the center of the 2, right where you placed your tweeter. It's not a coax, but it'll sound just like one. It'd be the cone area of a single 5.25 coax with more efficiency and have similar power handling too. The MTM arrangement will also beam in the vertical direction earlier, eliminating reflections off the dash up high.
For a 150db vehicle I'd strongly suggest running a pair in the pillars. If any 4's could keep up, these would be it, but IDK if I'd want to try it. It's just really pushing it on cone area and coil size. They also make a 3 incher that is just like it, which is on sale right now for 35. So for 140 dollars you'd have a pretty slick midrange setup using 2 3's per side.