I have a KTS-7, but haven't ever played a KTS-4. I would think that any difference in sound due to the extended fretboard would be pretty difficult to notice. Theoretically, yes, the fingerboard would have some effect on the top's vibrations, but I don't know if I would be able to detect it. I've played a Martin Style 3 (with the same extended fretboard) alongside my Martin Style 2, and I couldn't sense any difference in the way they resonated. The only difference I could tell from sound was due to different strings. Both tops seemed to be vibrating about the same as far as I could tell.
So, yes... the extra ornamentation on the KTS-7 is just that: Ornamentation. I bought one because I love the look of Style 3 Martins. The KTS-7 is a fantastic-sounding uke, and I can't imagine the KTS-4 would sound any worse... it'll just LOOK plainer.