Does using an unofficial charger negatively affect overall battery lifetime of a GS4? According to two employees at the Sprint store, using an unofficial charger for the Galaxy S4 will negatively impact the overall life of the phone's battery (edit: in other words, the battery will need to be replaced much sooner than if one always uses the official charger). 

I've never heard this before. Does anyone else know what they are talking about?  What is special about the S4 charger?  I saw someone online mention that a "special code" is transmitted over the USB connector, which the phone will use to judge if it is an official charger; again, new to me, but I'm not a hardware expert.

Update: I still haven't figured out an answer to this. Regarding the "special code", the post I saw may have been referring to the IC chip, but I don't know.  So if we look at the distinguishing characteristics of a Micro USB Charger, there's 1) whether it has the IC Chip; 2) whether it is 2 amp vs. 1 amp; anything else?
I shouldn't think so. Samsung is a signatory to the EU Common External Power Supply standard, which includes specs for a USB charger. As long as the charger you're using supplies something close enough to what the Samsung one supplies, you're good to go. You can even charge it from a Computer USB port, although it will be quite a bit slower (except some PCs which can supply a higher amperage via a micro-usb cable, making it almost as quick as a mains adaptor charger.)

But in any case, you should be fine as long as the charger is of decent quality and isn't going to catch alight or something. I've been mixing and matching HTC, Samsung, and Blackberry chargers for years now.