easiest way to measure the flow is to put a bucket on the output line and time how long it takes to fill a certain amount of volume. Not super easy if it's all plumbed up but a simple task otherwise. If it's all hard plumbed up making that task undesirable drill and tap an 1/8 NPT hole in the output pipe from the pump somewhere, then you thread in a pressure gauge to measure pressure to determine flow rate. You can screw a plastic plug in the hole when you're done.
If you really want to hit that kind of volume you might need two pumps. Hard to believe a skimmer needs 4000gph turnover and require that kind of power consumption. Sounds like a new skimmer would pay for itself pretty quickly
