Ok, thanks. One other thing, if you include just a pillar of an element do you have to use four of the underused cards as well?
Yes.
Does this also include having the mark or a pendulum of an element? (i.e Mark of Darkness with a Fire/ Water Deck)
Mark doesn't matter, but pendulums do. For example, you can have a Mark of darkness and 15 water, 15 fire cards, but you only have to include the unused cards for fire and water, but add a darkness pillar, pendulum, devourer etc and you have to use the unused card for darkness too.