I carefully outlined the two left-most figures and added some internal features. I drew the mustache on the next figure very quickly, along with the rest of the face. As for the fourth person, I drew him with tentative strokes, a totally different technique than I used for the first two portraits.

Two Sketched Figures and Two Sketchy Figures
Pen and Ink Sketch
10″x8″ 111# Moleskin Journal