How do you figure the flow into the sump?

[ReefNeck]

Now that makes it clearer.

If you go with a U tube type overflow box they will not lose their siphon when power goes off. I unplug all of mine for maintenance all the time and when I plug them back in they start working.

The CPR style overflow requires a pump or powerhead to suck the air out of the tube to maintain a siphon. I personally do not use this type because they fail more often than the U tube types do.

If your overflow can handle 600 GPH then get a pump that can do 650 GPH and with the 4' lift you will come out close to maximum flow without problems.

I hope that helps, If not....I'll shut up and let someone else try. lol

[AquaSquid]

Make sure that you place a check valve between your pump and the tank. this will prevent siphoning and a major flood in case of power outages.