well i had my nexus 4 crap out on me over the weekend ( i am on tmobile). friend of a friend was selling an unlocked Galaxy S4 (SPH-L720). looks like it was originally a sprint phone. if i turn it off and then back on i get the sprint splash screen. it is running on 4.4.2 and everything is working fine and dandy. i have my t mobile sim card in it and it works --calls--emails--everything. i did have to download a voice mail app to get voice mail notification icon working but other than that- works great!
i did download an app to check check if it was rooted ( root basic i believe it was called). phone shows it wasnt rooted. which leads me to believe it was sim unlocked. i do apologize as i am a noob at this --- but does the phone need to be rooted to do this? also i highly doubt it was carrier unlocked--so my main question is --- if there is an android update in the near future and i do update -- will it brick the phone? are there any updates i should stay away from as to not mess the phone up as it works great for now.