If I spend 4 or more energy to cast a spell on my opponent's turn, can I play Jhin, Meticulous Killer for his alternative cost during that turn?

Ruling: No. While spending 4 or more energy on a spell during your opponent's turn satisfies Jhin's alternative cost condition, you still cannot play him because you lack the timing permission to do so. Units can only be played during your own turn in a Neutral Open state unless they have a keyword like Ambush that grants special timing.

Nuances:

• Jhin's alternative cost is a replacement effect that changes what you pay, not a triggered ability, and it does not override standard timing restrictions.
• The condition lasts only for the current turn, so it expires at the end of your opponent's turn before you regain priority on your own turn.
• Only cards with Ambush or explicit Reaction timing can be played from hand during an opponent's turn.