The Force Unleashed does a good job of linking part 3 & 4: with the Dark side secretly plotting the Jedi's demise.
Bringing down the star destroyer should have been left out completely if you ask me, but hey ho. At the end, when your light sabreslast spark has been burnt, and its all done and dusted, there's a nice sense of achievement. But far from being put back in its box and left on the shelf for dust to gather, your find the game back on again, so you can see the the final upgrades at work, whether it being force blasting Jawa's through windows, or pulling Tie fighters of production rails, there's still things to be done. Various costumes, intriguing Jedi's you wont have seen before, Rancor monsters, its all worth a second crack at it, because by this time, you'll be the most powerful Jedi Master in the Star Wars universe.