Your problem could be interpreted one of two ways: either the slant height is doubled, or the perpendicular height is doubled.

Let's look our original pyramid, call it A. I'll assume you have a square-based pyramid. Then let each side of the base of the pyramid be b units, and the slant height (not the perpendicular height h units) be s units.

Your problem could be interpreted one of two ways: either the slant height is doubled, or the perpendicular height is doubled. If it is the perpendicular height, then pyramid A has volume

, and your 'doubled' pyramid B has volume

- therefore, it is four times bigger in terms of volume, and so four times heavier.

The harder way is if the slant height is doubled. Then, you end up with your volumes as

and

, which look a bit harder to work out, but the answer again comes out at 4. Therefore, I think your answer of 4 was correct.