I'd go for Rio. Visa is a bit of a hassle, but the city is incredible. Was there in July (middle of winter) and the temperature was still ~ 85 and sunny. Amazing beaches and great food. Stayed at the Sheraton for $90/night and they provided an excellent suite upgrade! For 4 days, I'd do 2 in Rio and 2 in Foz de Iguasu

CVG_JSG
SQ PPS NW Plat DL Gold Startwood Plat