Actually, you get more than double going from 4gb to 8gb, as the OS seems to take up about .75 gb. So the 8gb phone actually has 7.27 gb of usable storage, and I assume the 4gb phone will have about 3.27gb.
On my 8gb phone, I currently have 4.67 gb audio, 1.60gb video, 0.54gb photos, 0.03gb other, and .4gb free space. This represents 1110 songs or 2.9 days of music plus a few dozen podcasts, a bit under 6 hours of video, and about 850 photos.