If you have 2 astral heron does your second spell discount 4 energy?

Yes — both Herons trigger separately and both discounts apply to that same next card, so your second card costs 4 Energy and 4 any-domain Power less in total (FAQ #11679; Rule 356.4).

Each Heron's trigger is a separate object, so "when you play your first card each turn" fires twice, and cost reductions stack additively. Energy and Power just can't go below 0, so any excess is simply lost.

One timing caveat: both triggers must have actually resolved before you play the card. If a Heron trigger is still sitting on the Chain, that discount doesn't exist yet, and the reductions can't be split across two different cards either — it's one card getting the full 4/4 or two cards getting 2/2 each (FAQ #12624).