Discipuli is the nominative plural case (as well as the genitive singular and vocative plural but I won't go into those here). It is used as the subject of the sentence, the people doing the action. Discipuli libros habent -> 'The students have books' (The students are doing the 'having')
Discipulos is the accusative plural. The direct object of a sentence, often the people the action is being done to. Like the sentence here, quattuor discipulos habes, the students are 'being had'.
You can compare it to how most English pronouns change based on there use in a sentence. Compare "I like them" to "they like me". 'I' and 'they' are used as the subject of the sentence (like how discipuli is the subject) and 'me' and 'them' are used as the direct object of the sentence (like how discipulos is used in this sentence).
Hope that helps.
The speaker did say discipulUS, & I thought that it could not be correct. I wrote it any way as I'm new to Latin. It was marked as correct & yet I knew that the masculine nom. singular was not the same as the masc. accu. plural. I came here to the comments board & saw that, with my limited knowledge, I was correct. But now, if Duo marks the incorrect as correct, I'm lost!
In the feedback section there is a "My answer should not be accepted" option. Also, I recommend at the end of each lesson using the review lesson option to look at all the listening exercises, since when duo thinks you have "a typo" it'll be to busy telling you the translation to show the typo.