I play a 4-cost spell and want to play Jhin, Meticulous Killer by paying a blue Power Cost rune. The spell gets countered by Defy. Can I still play Jhin, Meticulous Killer?

Yes, you can still play Jhin, Meticulous Killer for his alternative cost of [C].

According to RiftJudge FAQ #10353, FAQ #9920, and the Rules FAQ Clarification 2026-04-29, Jhin’s requirement checks if the energy was spent and the spell became a "Finalized Chain Item" during the turn. It does not matter whether the spell resolves or is countered. Because you spent 4 energy to play the spell, the condition for Jhin's alternative cost is satisfied for the remainder of the turn.

Sequence:

1. Playing the Spell: You play your 4-cost spell. You pay the 4 energy. It is placed on the chain as a Finalized Chain Item.
2. Jhin's Condition: At this moment, you have satisfied the requirement "If you've spent 4 or more to play a spell this turn."
3. Reaction: Your opponent plays Defy to counter your spell.
4. Resolution: Defy resolves, and your spell is countered (cleared from the chain). Per Rule 425.1.b, the spell is not considered to have been "played" for trigger purposes, but per the Rules FAQ Clarification 2026-04-29, the energy you spent remains considered "spent" for the purposes of Jhin's alternative cost.
5. Main Phase: Because you are in your Main Phase and the state is Open (or you have Reaction timing if applicable), you may now play Jhin, Meticulous Killer for his alternative cost of [C].

Note: This is my interpretation based on the referenced official FAQ and Rules FAQ Clarification, which specifically address this interaction.