If you have a carrier locked Galaxy S4 you can sim or network unlock it using the instructions here but only if you’re using an older modem firmware. You have to downgrade to “ME2” to unlock it as Samsung have removed those menus. You can find older firmware roms here that you can flash using Odin.
It’s scary because you could brick your phone, but use the firmware checker to find the letter code for your phone (my international i9505 is XX) and search that roms page for “me2” to get the right one.
Why the urgency? The latest update from Samsung that came out a few days ago locks things down tighter. Android 4.3 is supposedly coming in October but I have it on my Nexus 7 and the good stuff is in the Play Store apps. It might be worth disabling the automatic update checker (System Settings->More->About Device->Software update) just in case you update by accident. If only I didn’t like the Samsung camera app so much I’d put the Google Edition rom on there..