In the sentence "Ihana sleeps only for four hours a night," the verb 'sleeps' is intransitive because it is a verb that does not require a direct object to make sense; 'sleeps' here describes an action that Ihana performs without acting on something else. 
Here's a breakdown of why:
Intransitive verbs
describe an action that an individual performs on their own. For example, in "Ihana sleeps," Ihana is sleeping, but she isn't sleeping something else. 
Transitive verbs
require a direct object to complete their meaning. For example, in "The apartment sleeps six," the hotel is accommodating six people, and "six" is the direct object. 
Therefore, because the verb 'sleeps' in the sentence about Ihana does not have a direct object, it is intransitive.