Yes Don it's time to reinforce that and confirm that we count to three rather than four. I seem to remember Cicero questioning this in On Ends, and perhapes Aulus Gellius defends Epicurus, on the same point:
1 - Natural and Necessary
2 - Natural but not Necessary
3 - Necessary but not Natural ???????
4 - Neither Natural Nor Necessary
If you have Cicero's citation handy, that would be handy.
That's one reason I like natural, necessary, and empty.
Epicurus' categories are not a strict permutation of the words un-natural and un/necessary.