The stand-alone adventure Troubles In Otari, provides the advancement information necessary to take characters created using the Beginner Box rules from Level 3 to Level 4.
While not part of the Beginner Box material, pages 56-59 are formatted identically to the advancement pages in the Hero's Handbook, and allow a final, fourth level of play before a player needs to switch to a standard character sheet, and the full rules in the Core Rulebook.
The entire adventure can be played using only the BB Hero's Handbook and the Game Master's Guide.
