If you imagine the triptych of an altar, the centrepiece is twice the with of each of the wings. When these are both folded in towards the centre, the central altarpiece is completely concealed. This is the derivation of the term. It IS a form of gatefold, but gatefold covers other uses as well such as the large pages, sometimes inserted into books, which can be folded out (maps are a prime example of this). "Altar-fold" is certainly used in the UK printing trade. But how page 4 comes to be the LEFT-hand fold is a mystery - my memory of these things fades rather, but to me that would be page 1....