"BenignVanilla" writes:
So 4'' for my bottom drains, skimmer, filter, all runs?

Note that 20 feet of 4 inch pipe holds 131 gallons of water.
That's as much as three 50 gallon barrel filters typically hold.
If this is above the water line of the pond, and if it's a small pond,
it'll lower the pond after the pump is turned on. And if power is
lost, it'll drain back through the pump and overflow a small pond.