Thanks for the response, @AlexColgan!
Sorry, I meant older versions of the Leap Controller hardware. I haven't been able to determine whether there are any other versions, so maybe that isn't an issue. E.g. if I write software with API V4, would it run on a Leap Motion Controller someone purchased back in 2013?
Brilliant! This was my main concern. Sounds like V4 is the way to go. Thanks again Alex!