The problem with 4-down towing is heat in the drivetrain and inadequate lubrication. Keeping the speed down is one way to alleviate those. Chances are that exceeding the recommended limits won't have any noticeable effect in most situations, but at some extreme of air & road temperature, driving conditions, etc. the transmission, axles and 4WD transfer case may get excessively hot. Whether that means immediate breakdown or merely a somewhat shortened lifespan is open to conjecture.
FWIW, I towed a Chevy Tracker with a 55 mph and 200 mile towing limit for over 30k miles, ignoring the 55 in favor of 60-65 mph. We ran the engine at 200 or so miles if it was convenient to do so, but never made stops just for that purpose. Maybe it shortened the tranny & transfer case life, but I was never aware of it during the 55k miles we owned the vehicle.
