A lot of good information provided by msinger. Highly recommended reading before you buy. An excerpt below from an interesting Powerextra battery review from an Amazon user could shed some new light on the subject of unexplained in-flight power failures for P4 series models that keep popping up in this forum. Smoking gun? Maybe not but, then again, maybe it should be a talking point.
I've purchased five genuine new DJI batteries in the box on eBay for nothing over $85. For me, risking a very expensive flying camera just to save a few bucks more on an aftermarket battery isn't a risk that I'm willing to take. Just my 2 cents.
I have found on occasions its tricky to have the battery click into place on the drone. Upon closer inspection, I noticed the points (see photo) have become mangled from adding and removing the battery from the drone. Ultimately that means there is a chance that the flow of power from the battery to the drone in mid-air might falter. Not worth the risk losing a $1500 drone with these batteries. I have 3 of these batteries and the problem exists on 2 of them. I have had 3 of the DJI phantom 4 batteries and this problem does not exist.