Since you're traveling for a short period of time you may be better off with using WiFi. If need be for emergency purposes you should use your phone then. The Nexus 5 is good for using different networks since it is unlocked by factory. But in this instance you should just use WiFi. Hope that helps. :)
Enjoy the trip (I'm from Sarajevo and am quite familiar with the region).
You'll be visiting 4 countries so you'd actually need 4 local SIM cards. That may be a hassle unless you really need to stay connected. I don't know if it's worth it or not because I don't know how long you'll be there for (I don't know if you're leaving in 4 days or if you'll be spending 4 days there).
But if you do, I recommend HT Eronet in Bosnia, Tele2 in Croatia, and Vip in Serbia. I haven't been to Hungary yet, but I'd still recommend Vodafone there based on my experience with Vodafone elsewhere.