It's most likely the blower resistor ( a very common problem).
However the way this circuit is configured, if the blower works on on the 2nd position, it should also work on each of the positions higher than that. In other words if #2 is the first speed to work on your blower, then #3 and #4 should also work.
If #3 was the first speed to work, then so should #4.
Because of this discrepancy you may have a problem with the selector switch itself which is a much less common problem.
All the same I think you should check the resistor pack first since it has the higher failure rate.
I had similar problem and replaced heater switch cost about $40.00 in Winipeg Canada. 3 years ago replaced it (heater switch) and now switch not working med high. Last time replaced in summer think I will wait till summer again.