They'd probably look wider, but also probably shorter, maybe even TOO short. I have the same tires on 15x4 Convo Pros and I think they look just about right in regards to width/height, but like you said, they're substantially narrower than the rears....I have 28x11.5x15 QTP's on mine.
I'll be working on the GN this weekend, so if you want, I'll snap some pics of 'em. Probably not a whole lot different than how they'd look on a 15x3.5 though.
Steve