I thought I would share my experience regarding fake Canon NB-4L batteries. Let me start by saying I own a Canon IXUS 55, with the girlfriend owning a Canon IXUS 40 both using the Canon NB-4L Battery. I have purchased two spare batteries from eBay, with one of the purchased batteries looking to be fake. The following photos show the four batteries with the differences circled in red and reasons for this belief.
Firstly, a front photo of the four batteries, the lower right battery is the fake. Circled in red is a single linear indent on the fake battery whilst the real batteries have two linear indents. The text on the fake is also quite blurry.
Secondly, a back photo of the four batteries, the lower right battery is again the fake and again the single indent can be observed on the fake. Circled in red is the caution text which again is blurry. It can be observed that the words on each battery are different, and hence I believe is not a good indication to its authenticity.
Lastly, a isometric view of the four batteries, the lower battery is the fake. Circled in red is a little rectangle, with the genuine batteries having a rectangle piece of plastic stuck on while missing on the fake battery. This would have been to save on manufacturing costs.
There are other differences, but those above are the most obvious. Click on the above images to display high resolution photos.