23476

Each piece of paper is defined such that when two peices of paper are put together they become the next size up, where the ratio of the sides is the same.

If we have a peice of paper with dimentions x by ax where x<ax therefore x is the shorter side and ax is the longer side. When two are put together it forms a peice of paper with dimentions ax by 2x, where ax is now the shorter side and 2x is the longer side.
Hereby we can see that for the definition of this type of paper to be true it has to follow that x/ax=ax/2x, multiplying both sides by 2a, a^2=2, therefore the longer side is exactly root 2 times as long os the other.

Also since A0 is defined to have an area of 1m square, A4 must have an area of 625cm square so sqrt(2)x^2=625