The fourth crusade was meant for Jerusalem but they didn't have the money for transportation so the crusaders struck a deal with the Venetians to reclaim Zara for them. Basically, during this time Alexios, who saw himself as the rightful heir to the throne in Constantinople, went to the crusaders and asked them for help to reclaim it and made Pope Innocent III all sorts of promises. So this became the new plan and was somewhat successful. The problem was that when Alexios reclaimed the throne he was not able to live up to his promises. The Byzantine empire had been weakened over the years. The people became resentful of Alexios and rioted and as a result Alexios Doukas became emperor and he established defenses against the crusaders, who had had enough at this point and laid siege to the city. They repelled the first attack but not the second.

There are many theories that suggest that Pope Innocent III and the Doge of Venice, as well as some of the crusaders, may have been a little too eager to attack Constantinople, to take advantage of the weakened empire for personal gain or revenge. For that I would check this out. http://www.reddit.com/r/AskHistorians/comments/24ng5x/fourth_crusade_conspiracies_please_explain_them/