April is a pleasant month to be in Santiago. I echo the recommendation for a light jacket. The sun will be nice and strong during the day, but as soon as it goes down, it will turn a little chilly.
When I lived in Santiago, the rains didn't start until mid-May, but I understand that it varies from year-to-year.
Note that temperatures will be 5-7 degrees cooler (Celsius) if you decide to visit Valpo/Viņa del Mar. Definitely need a jacket there.