What do you call a quarter of an A4 sheet of paper?

Answer 1

A6.

The A series of paper is such that the next number up is half the size of the current number.

Thus 1/2 A4 is A5, 1/2 A5 is A6.

And 1/4 A4 is (1/2 (1/2 A4) which is 1/2 A5 which is A6

Answer 2

A quarter of an A4 sheet of paper is typically referred to as a quarter sheet or quarter page. It is a common measurement used in printing and design to indicate a smaller portion of a standard A4 size paper.

Answer 3

Half of A4 size paper is size A5.

Same as A4 is half of A3.

etc. etc.