I think someone could very well justify going for the 4gb if, - just imagine! - they had 4gb or less of music. A random 4gb selection of my mp3 library (192kbps) yields 799 songs - 67 full albums (if you take an album to typically be 12 tracks), which is around as many "physical" albums as I, for example, own and I expect more than most casual music-fans have. Combined with the smaller size and weight, design which will certainly be more appealing to some people. and the inclusion of the dock and USB 2 cable (15 gb doesn't have these does it?), I think there is far more between the two models than just the capacity.
Ignoring the fact that personally I think the mini is really bloody ugly.
edit: for broadband's benefit, my feeling on the mini is: very dodgy proportions (spaces between the wheel and the screen and sides for eg), combination of curved and square edges, minging colours (I'm sure they will appeal to some people, but not me), and a basic design concept which is essentially the iPod reformatted/rescaled to suit a substantially different device, and not in a complimentary way. It just doesn't seem to fit anywhere in Apple's design regimen.