How to earn chests?

I've never earned a chest on Sona (says so when I hover her in my collection) and I was playing today and one of my teammates got an S on Heimer. However, I did not receive a chest after the game. I am really confused- do I need to get an S or can anyone in my team just get an S? Also, when I check my profile it says 4 under the little chest icon.
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