I wouldn’t call the fourth crusade “what truly led to the demise” of the Byzantine Empire.

The Byzantine was a waning state that had been shrinking since roughly the late eleventh century due to the rise of Islam and the Seljuk Turks. Following the Battle of Manzikert in 1071 AD the Byzantine Empire löst much of its territory in Asia Minor, and continued to shrink towards that point on.

It is important to note that the Byzantine Empire persisted for around two centuries following the sacking of Constantinople.

The Byzantine Empire faced major issues, such as an over reliance on foreign soldiers and mercenary armies, a reliance on an inefficient and ineffective Theme system, and actions by crusaders before the fourth crusade. As early as the third crusade, many Christian crusaders pushed Muslims out of formerly-Byzantine lands, forming the Crusader States. The fourth crusade was certainly integral to the demise of the Ottoman Empire, but was one of many nails in the coffin. (And we should strive to avoid oversimplifying the decline of empires to one singular event, as no civilization’s decline can be attributed to one key source.)

Minor point: Hopefully this does not come across as rude, and did answer your question! English is a second language, so I hope I also made sense.