What could be the length of the third side of a triangle that has two sides of lengths 4 and 15?

For a triangle to exist, the sum of the two shorter sides must be longer than the longest side.

If 15 is the longest side, then the other, missing, shorter side must be greater than 15 - 4 = 11.

If the third, missing, side is the longest side, then it must be less than 15 + 4 = 19

So the third side is any length greater than 11 and less than 19.

Examples include 12, 13, 15, 11.5, 18.5.