The 4th crusade screwed Constantinople big time, is this the reason why the ottomans were able to capture it, and if the 4th crusade didn't screw everything up is it possible that the ottomans never came in europe?

Hard to say, as the arrival of the Ottoman took place 150 years after the sack of Constantinople and the Ottoman needed another century to take the city.

But we can consider the 4th Crusade to be the "coup de grâce" to a declining empire. The failure of Manuel I Komnenos to restore the power of the Byzantine Empire (the usual explanation is that he focus on the distant dream that were Egypt and Italy, instead of trying to reconquer and suppress the immediate Seljuk threat in Anatolia; when at last he try to repel them, it was too late) and the terrible rulers that followed him doomed the empire.

The administration was really inefficient and corrupted, the central authority collapsed, while local magnate increased their power (Serbia and Bulgaria even declared secession from the empire!).

The empire is also weakened by external and civil war: Albania, Macedonia and Thrace are ravaged by the Normans of Sicily. War against Hungary, Serbia and Bulgaria. War against secessionist from Lydia. War against the Holy Roman Empire, the emperor Isaac II did a poor choice by allying to Saladin and opposing to the passage of the crusaders of Frederic II Barbarossa. The treason of the Byzantine empire is one of the reason of why the west would hate them and one of the reason of the 4th crusade disaster.

At the time of the 4th crusade the empire had no more decent army, they only rely on mercenaries, likewise it had no more a war fleet and hence had to rely more and more on Genoa and Venice, giving them increasing privileges to the detriment of the byzantine merchant (which would again weaken the empire and increase the influence of the italians merchant republics).