Personal preference and style. The reason to wear a yarmulke invokes no need to have any particular style. There are those who have associated yarmulke styles with belonging to certain communities but this has nothing to do with the intent of wearing a yarmulke to begin with and is only a social construct created after the custom of wearing a yarmulke became universally accepted by Jews.
To show that the reason for wearing a yarmulke does not indicate whether one should have one made up of 4 panels, 6 panels, or any number of panels this is the reason for wearing a yarmulke: A yarmulke is made up of the words yar, mul, and ka (Fear the regime of Hashem). This is to say a Jew who wears a yarmulke is expressing the desire that they should always have the fear of G-d with them wherever he goes. This means that he fears feeling distant from G-d as a result of negative actions which he then won't take. One is reminded of this fear of G-d by wearing a yarmulke always.
