When having Revna the Lorekeeper on board, it says that if we play a spell, if we spent 4 or more, I can ready it. Does the "spent 4 or more" part refer to the spell cost, or can I play a 0 cost spell and ready Revna as long as I spent 4 or more during my turn?

To determine how Revna the Lorekeeper's ability works, we look at the exact wording: "When you play a spell, if you spent [4] or more, ready me."

The "spent [4] or more" refers specifically to the energy cost of the spell you are currently playing.

Here is the breakdown of why this is the case:

1. Ability Timing: The ability triggers "When you play a spell." This is a triggered ability that finalizes as part of the process of playing that specific spell (or immediately after).
2. Tracking Spent Energy: When you are in the process of playing a card, the game tracks the resources spent to play that specific card. Since the condition is tied to the act of playing the spell, it looks at the cost paid for that spell.
3. No "Turn-Long" Tracker: The ability does not check your total energy expenditure for the entire turn; it checks the cost of the spell that triggered the ability.

What this means for your scenarios:

• Can you play a 0-cost spell and ready her by spending [4] elsewhere? No. If you play a 0-cost spell, you have not "spent [4] or more" on that spell, so the condition for the triggered ability is not met.
• How do you trigger it? You must play a spell that has a total energy cost of at least [4] (after applying any cost increases or reductions) and successfully pay that cost.

Note on Cost Reductions: If you have effects that reduce the cost of your spells, be aware that the condition checks the amount you actually spent. If a spell has a base cost of [4] but is reduced to [3] by an effect (like a discount), you have only spent [3], and Revna will not ready. Conversely, if you choose to pay optional additional costs (like [Repeat] or other added costs) when playing a spell, those costs are part of the total energy spent on that spell and will count toward the [4] energy requirement for Revna's ability.