The response to 4X will continue after the manager stops after 21 to 42 days after calving. Milk yield may drop slightly, but the lactation curve will remain higher. If the herd is 3X, then they need to milk 6X.
Illini DairyNET
Milk yield after the cows return back to 2x milking from 4x milking has usually been reported to be 6 to 8 pounds higher per day compared with similar cows milked 2x throughout lactation.
Dave Wilson, Utah State University Dairy Extension Veterinarian