Three of the following four letter-clusters are alike in a certain way and one is different. Pick the odd one out.

XTZ IFK PLR GCI

IFK

XTZ ⇒ X→(-4)→ T→(+6) →Z IFK ⇒ I→(-3)→ F→(+5) →K (ODD) PLR ⇒ P→(-4)→ L→(+6) →R GCI ⇒ G→(-4)→3→(+6) →I