Are you serious!
4000 gallons is on the table! Money can not be a concern!
(I'm just saying!) no fights!
Even rock/sand is going to be more than some peoples entire fish hobby lifespan! This is nuts!
I'd suggest a gas boiler of some description but the trouble with salt water and metal makes it tricky.
You could heat the room, that's what I do, then push individual tanks as appropriate. I used an under cabinet kitchen heater rated at 1 or 2 KW (switchable) and a reverse thermostat which keeps me at 22C if it gets cold. Insulated walls. Humidity control fan. Or in this case, perhaps fans and heaters!
It will handle 28c easy in my room and it's only on the 1kw switch.
Maybe you could insulate the concrete element, kingspan in the walls maybe?
And plenty rebar!
It'll be 15 ton of water! (34,000lb I think!)