What fastener to use to attach 4" circular pan to steel stud?
If the steel stud is of sufficient thickness/strength you could fasten it with self-drilling Tek screws. I would personally be more concerned with the weight bearing capability of the steel stud (assuming it is the lightweight steel studs commonly used in commercial build-outs in my area). 

Most ceiling fixtures (you use the term stud rather than joist so I am assuming wall mount) that are expected to hold any weight should be attached to a cross-brace which is attached to two joists. 

Consider using a long pilot drill and drill through both the front and back sides of the steel stud, then use a long self-tapping screw to penetrate both sides of the stud.