How to introduce Kotlin in a Programming Languages course
I think the answer to that depends on how you're structuring the course overall, and what Kotlin brings to the teaching of those ideas.  I know of three distinct approaches to an upper-division PL course:  the "paradigms" curriculum (à la Sebesta's book), the "interperter/design space" curriculum (EOPL, Scott's "Programming Language Pragmatics", Louden, Turbak/Gifford/Sheldon, etc.), and the hybrid theory/implementation curricula (less common and few things in print: the Racket team, Norman Ramsey's adaptation-in-progress of Samuael Kamin's book).

Do any of these describe what you're doing with your course?  What is it that Kotlin brings to the class that is not already represented by the other three functional languages your students have learned?