Do you know your coolant pump drive ratio? You can find it by dividing the diameter of your crankshaft pulley by the diameter of the water pump pulley.
8” crank /6” pump = 1.333, or 33% overdrive for the pump. 4000 engine RPM would be about 5300 RPM at the pump. A figure less than 1.0 would indicate an under-driven pump.
6” crank /7”pump = .85, or 15% under-drive. 4000 engine RPM would be about 3400 at the pump.