A centrifugal pump has variable output depending on the head. There should be a pump curve that you can reference to find that number. Some portion of that is lifting water. Keep in mind that there is more to it than just from the discharge to the pond. One must also include lifting water to the pump itself on the suction end. This has its own curve. As a general rule, keep the discharge and suction conduits the same diameter as the those on the pump. Restricting a 4" pump to 2" will greatly increase the head resulting in lower flowrate. A lower cost conduit that would probably work well with a 4" pump is the flat irrigation tubing.
Its very affordable for long spans.
Don't overcomplicate this. You have a reference on your 2" pump at 75 gallons per minute. A 4" pump that has similar head limit will produce ~4x that rate provided you give it 4" conduit. It will probably not be worth putting 4" hose on a 2" pump though it would increase flow by reducing the frictional loss. Were it me, I would make the decision based solely on the benefits of pump size and size the conduits accordingly.