Every time you reach a level divisible by four, including four in any given class, you get to pick either a feat, or adding two points to your ability scores. It is worth remembering that this is not every four levels, but every fourth level in a class. So if they add multiclassing, you could reach 3 fighter / 3 rogue and have never gotten a feat or ability score increase. The other major exception to this is that fighters get this every fourth fighter level as well as sixth, twelfth, and eighteenth (I believe).