I was wondering the same thing earlier today...after several google searches, I finally decided to pay a little more and order the top of the line replacement from iPodJuice.com. The battery I ordered is 1200 mAh and is supposed to last 2x as long as the original 4G battery.
Other sites had cheaper batteries, but the highest capacity I could find (excluding iPodJuice.com) was 1100 mAh.
Another reason I went with iPodJuice.com was that the quality of the tools they supply to open the iPod case seemed to be of much higher quality than the competitors.
I should have the battery in a few days; I'll repost with my impressions.