I think go 8 GB and ditch 4 GB. I also doubt it will go color, yet. Look at every other iPod. Their width increased so much after going color. They are wider than the 3G 40 GB iPods and those were like bricks. The mini will need to wait a year or two to go color. BUT, I think the minis price will increase about $30 for the update. So, instead of $199 buying you an iPod, it will be $229 for a 6 gig and $279 for an 8 gig. But the power adapter will go down in price to $19, so when you buy a 6 gig w/ adapter you pay ~$250 and for an 8 gig w/ adapter ~$300.
Also, with the mini losing the 4 gig version, the shuffle will get a 2 gig version, but also keeping the 512 and 1 GB. SO, for $99 you get 512, for $129 you get 1 gig, and for $179 you get 2 gigs.