Which word is used when bowler takes 4 continuous wickets in 4 balls? (such as hat-rick for 3 wickets in 3 balls)
It is Four in Four according to Wikipedia.
Taking two wickets in two consecutive deliveries is occasionally known as a brace, or (more commonly, especially until the next delivery has been made) being on a hat-trick. Four wickets in four balls is referred to in cricket literature and record books as four in four but the term double hat-trick has also been used in the media, as it will contain two different, overlapping sets of three consecutively dismissed batsmen.
Note : Cricket does not define an official term for 4-in-4.