Apologies if this has been asked a 100 times already, but has the four letter first name thing for the humans ever been explained, or is it just apart of the 413 thing?

Yes.  The kids’ first names are 4 letters long and their last names are either 6 or 7 letters.  6+7=13   The human naming scheme is a reference to the arch-number of homestuck 413.  Similar to how at the beginning of the story it was about 4 13-year old kids.