Oh man. Apparently Apple has been buying up flash memory and some are speculating they will launch an 8GB nano soon.
While I miss those extra GB dropping down from a hard-drive based iPod, I've been in love with my nano since it first arrived last September. It's an amazing little device, and I prefer 4GB wherever I go than 15GB that I may or may not be carrying with me.
That said, 4GB is a bit tight... I have to rotate music a lot and I don't carry around as much music that I listen to every once in awhile. Of course, higher-capacity flash players are inevitable, but the jump to 8GB would mean a lot more for me than the jump to 6GB.