A:
Samsung is releasing an unlocked version of the Galaxy S4 running pure Android 4.2 without any additional software in June. It will be a GSM based phone (requiring a SIM card) which should work in Brazil as well as on the two GSM US carriers, AT&T and T-Mobile. The announced selling price for that phone will be $650.
The T-Mobile version of the Galaxy S4 and the AT&T version being GSM phones which use a SIM card MAY also work in Brazil especially since AT&T is a local cell provider there. FYI, VIVO, the largest cell provider in Brazil still has both CDMA and GSM networks in their system depending upon where in Brazil you will be located.