They aren't just sister companies. They use exactly the same digital boards in both receiver lines (although different boards at different price points). Only the analog circuitry and logos differ. I'd expect that the increase in audio quality provided by XT32 and SubEQ in the X4000 would be noticeably better than what's provided by the slightly better analog circuitry with only XT provided in the Marantz receiver.
FWIW, the X4000 is not the top of Denon's receiver line. That's still last year's AVR-4520.