Why do you need more than 4GB? I can't imagine a need for more space than that (at least not one that can't be better serviced some other way).
If you want to carry around your music, wouldn't an MP3 player be more convenient? If you want to carry around lots of large files, a portable hard drive/USB stick would be better.
If you want to carry around several thousand books that you want to access at any time, then I suppose keeping them on the Kindle is the best way, but that's probably not a common enough usage to make Amazon increase the Kindle's storage.
Perhaps getting a tablet computer instead of a Kindle would be more versatile for your needs (tablets allow USB input, memory cards, etc. to be added).