The book I'm reading essentially summed up the Byzantine forces as inept and cowardly during the time of the fourth crusade, thus allowing the numerically inferior crusaders to succeed. Is this true? Why were Byzantines in such poor conditioning?

As it is with many woes of the later Byzantine Empire, the answer is "blame the Angeloi."

Following the overthrow of Nikephorus and the ascension of Alexius I, the Byzantine army was wildly reorganized from scratch. The Byzantine Army "became" a force made up of central troops (The guards such as the Varangians, although older forces such as the Scholai died out) that was complimented by "domestic mercenaries" called the Tagmata (these were full-time professional soldiers, and a departure from the earlier Themata system of part time soldier-farmer levies), along with the personal retainers of the Dynatoi (semi feudal magistrates that governed the provinces). These troops would assemble under the command of the Emperor, the Grand Domestic, and the Protostrator. During the Komnenian period (1080-1180, so right before the fourth crusade) this system basically placed the entirety of the Army under the authority of the Emperor and his immediate family (who would be given the aforementioned high positions ), and Alexius, John, and Manuel Komnenos all took great care in personally selecting who would lead what. The issue is that this caused a great deal of reliance on the Emperor himself, which is fine when you're a member of the Komnenoi dynasty and your genetics are basically pure gold, and this is reflected in the Komnenoi arguably being the most successful dynasty in Byzantine History. However, following the rule and overthrow of Alexius II and Andronikos I, this system broke down. Alexius was too young to do anything and Andronikos spent much of his reign trying to break the back of the landed nobility, the people who provided him much of his troops and competent commanders.^1

Following THEIR rule, you have the Angeloi, who's rule saw the breakdown of the Byzantine state due to civil wars, general incompetence at ruling, and the general collapse of the Byzantine economy and treasury (Alexius III had to distribute so much gold to get people to support him after his blinding of his brother, the one relatively competent Angeloi Isaac II). Their rule saw the breakdown of this system and the fostering of large amounts of mistrust between themselves (and their magistrates) and the landed nobility, leading to a breakdown of the finely tuned military machine of the Komnenoi. This breakdown of effective leadership led to a breakdown of effective training and discipline (along with soldiers not being particularly fond of the emperors), and the overall military incompetence that your book is alluding to.^2

[1]The Development of the Komnenian Army: 1081–1180, John Birkenmeier

[2] The Byzantine Empire, 1025–1204: A Political History, Michael Angold