I'm headed to SCL in a couple of days..... I believe there is indeed 4G in the city based on my research. I use a ton of data and no voice or texts (I use iMessage and WhatsApp which are both data). Based on what I know now I am going to buy a Movistar SIM at the airport and get the most amount of data for a one week visit. Happy to answer any questions before/after I go. Here is one site I used for research....
http://prepaid-data-sim-card.wikia.com/wiki/Chile