Don't be scared... this iPhone 4 battery CAN be replaced!
5 - 30 minuten
The battery on my wife's iPhone 4 was not behaving like a good battery... it was being a bad battery and not staying alive for very long during the day with normal use. After talking it over... we decided that the old battery needed to go.
Followed the iPhone 4 battery replacement guide exactly as it is presented to get the old battery to leave the iPhone and welcome home a new battery.
Round-trip, it took around 15-20 mins. being SUPER careful (I didn't want to muck it up and have to explain to my wife that her iPhone didn't make it through the battery surgery).
HOLY GLUE! Don't even bother trying to kick the old battery out of your iPhone with the thin plastic battery tab... there was a TON of glue holding it in there. Had to resort to using the plastic opening tool to loosen the battery from it's place (SUPER carefully, of course).
Other than that... the repair was very straight-forward.
BE CAREFUL when connecting the new battery harness to the iPhone... make sure the cable is "pre-bent" to allow easy installation.
I used the plastic opening tool for all of my prying / separating work on the iPhone so I wouldn't have to touch things with my fingers and clean the metal contacts like outlined in the guide.