I too have Forte 3. Have heard IV but not side by side. Agree with the other poster about liking the base of the 3 as opposed to the 4. I contacted Klipsch when I heard about the kit and they had no info but said someone would contact me. Of course I never heard back.
What I find interesting is that if one goes back and reads forum threads and reviews on the 3, virtually none of the current criticisms of the 3 ever occurred. In fact it was mostly rave reviews, sans the general criticism of klipsch overall from some (too bright). It seemed like those didn’t really occur until the IV was released, and then mostly from many dealers saying various negative things about the 3. Call me skeptical as accepting these criticisms from those who want to sell something else. Like many others, I struggle with finding (or accepting) “good enough”. As an experiment, I could haul my 3s into my local dealer (who has 4s), and do a blind side by side (3 and 4), but that’s a lot of hassle. It may very well the upgrade is worth it. I’d like to hear from others who have done it to see what ultimately the differences might be. And then, if other things could possibly improve the sound - such as is it more of a matter of personal taste and a variety of factors such as amplification, or switching to tubes if one is running SS.