Aktualizuj tylko czas z mojego pola Datetime w sql

UPDATE MyTable 
SET MyDate = DATEADD(HOUR, 4, CAST(CAST(MyDate AS DATE) AS DATETIME)) 

Albo to

UPDATE MyTable 
SET MyDate = DATEADD(HOUR, 4, CAST(FLOOR(CAST(MyDate AS FLOAT)) AS DATETIME))