I think it shouldn't be easy to gain access to four ship slots. I believe it would work well in a quest chain, after you've hit level 50. I also digress, and believe that many quest chains should be introduced at 50, and perhaps a heightened level cap soon after those are introduced. Perhaps even more of the main story, to allow for that addition on to the level cap.
Back to the main topic: If it were introduced, it would be good coupled with questing or other activities (read: earned).
Christopher