Say I have a data class that accepts an item, that item could be one of say 4 different things. I know I can use the Any type but what's the best way to narrow the scope to those 4 items?

Until we have union types, your best bet might be to replace your data class with a sealed class/interface that has four distinct subtypes

I vote for sealed interface the 4 types would implement

a nice advantage of sealed interfaces over sealed classes is that data classes can implement them.