Hello, I am also confused by this problem.
Actually, this is not a bug. Because “et al.” mean “and others”. Note it is plural. If you set the rule as:
4 or more authors, show first 3 and then et al. So et al. means only one author. This is an error. Endnote 21 will correct it and show all 4 authors.
I hope Endnote can give this decision to customer self.