It used to be that April signaled the first rains, but they seem to come later now. You might get a bit of rain. It is difficult to predict weather anywhere in the world now, but at that time of year the temperatures should be in the high teens to low 20 celsius range. A light jacket is usually enough during the day and even in the evening. Do you know where you are staying? Here is a website about the different neighbourhoods in Santiago which you might find interesting:
http://www.allsantiago.com/