J4 is not marked and there are not 6 but 5 dots. Just left of the Arduino logo. They are in a 2x3 configuration, one middle pin missing.
You can download the Eagle ZIP file and then drop the *.brd file here to look a the signal traces.
https://www.altium.com/viewer/
Search for SWC and SWD to get the to serial wire debug lines. The other signals are 3.3, GND and Reset.