Does using an unofficial charger negatively affect overall battery lifetime of a GS4?
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.