How do you get by level 4 in diablo

Level 4's exit may be blocked as part of a quest called Ogden's Sign. A Dark One will talk to you and demand you get the sign back from the 'big uglies' just to the north. After you head there, and defeat the group of Overlords, you can retrieve the sign. Whether you give it to the Dark One, or return it to Ogden for a reward, the way down to level 5 will be unblocked.