New mod on 4*60 (Have Pics Now)
I don' t think you'll see any change in performance.
The only thing I could picture your change affecting would be something that relies on airflow being stifled as it "tries" to flow around the flat-sided fuselage -- so knife-edge might suffer, but the 4*'s not really a great knife-edge plane anyway is it? The plane may have a little less drag with the rounded sides, but I really doubt that it would be enough to notice.
While you're at it, you can probably also change the shape of the tail and wingtips without affecting performance (if you have a mind to). Just keep the same areas and incidences. A nicely kit-bashed 4* can produce a better flying sorta-scale version of an awful lot of low-wing full size planes, too.