What's the number 4 under the root sign?

2。 Because the square of 2 equals 4, 4 under the root sign is 2. In mathematics, the root sign means the square root operation of a number, that is, finding a number so that the square of this number is equal to the squared number. In this problem, we need to ask that the square of a number is equal to 4, and this number is 2.