There is a whole post with partial Kindle 4 disassembly over at blogkindle.com. It reveals 890mAh battery along with ample empty space around it. New version of Atheros WiFi chip and 512kb of flash memory. Read the post to learn more and see pictures. All-in-all I wouldn’t call Kindle 4 battery “user-replaceable” but in all likelihood you are not going to need replacing it. I still have Kindle 1 and it still runs on original battery just fine.