Actually I believe the 4 prong has a common whereas the 3 prong does not. The common is used with either hot wire to get 120v (like in a stove) whereas the 3 prong only has two hots so you can only get 240v (like in a dryer)...
Unless you bring that connector into a panel, in which case you can get 120 between the neutral and either of the hots.