HK-47 and Bastila's quests will no longer be available if you haven't completed it yet. Second, Dantooine will no longer be available to you and so any side-quests that involve Dantooine cannot be completed (including part of the Genoharadan side-quest and the ending of the Stowaway side-quest). Make sure all characters that you will be using are fully equipped (this includes your PC, Bastila and Carth), and that all others are not equipped with items that may be vital later on. You don't have a choice in the matter, so make sure you are prepared beforehand.
Read this section before proceeding! Firstly once you have the fourth star map (including Dantooine) you are automatically taken to the Leviathan the next time you Leave Port. WARNING! Visiting the Leviathan will trigger certain events to occur which cannot be undone.
