Please help...I want to be clear about Ich treatment

I knew all the fishes should stay in QT for minimum 4 weeks to treat ich by hyposalinity after the white spots disappear on the fish's body. So my question is why are we need to wait 4 weeks after those fishes out of white spots same mean the fishes out of ich? Can I move those fishes to other QT with setup hyposalinity as same as the 1st QT. The 2nd QT won't have any ich drop out from fish so I don't need to wait other 4 weeks to kill ich in the QT.

Please give me some answer to clear my mind.

Thanks,