---Sent from the most advanced android mobile super-computer on the planet: the Samsung Galaxy S3 on AT&T, featuring simultaneous voice & data and speedy 4G!!!
You had the screen on for all 20 minutes. Considering that, 4% drain is totally normal.
Also that's your first 4% which means you might not have started at 100%. When you take the phone off the charger it might immediately drop to 98 or 99% due to smart charging protocols.