For an element to be considered "used" in a deck, there must be at least 5 non pillar/pend/mark cards present in the deck.
Does this mean we can use up to 4 cards from any element or only the elements of our season?
Can we use pillars/pends of an element from a different season? (Ok that question is probably covered with "marks/pillars, pends can be used freely")
Nope. It means that you need to include at least 5 (so 5 or 6, as pillars/pends/marks don't count for this), of in-season cards, for every element present in the deck. For example, you are using Summer, and want your deck to be legal for the elements
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. You must include 5-6 non pillar/pend/mark cards from Fire, and 5-6 non pillar/pend/mark cards from Time.
Yep.
Hope that clears everything up!