Is there a fourth word in this series: weekly, biweekly, triweekly, ...?
If not, and I had to coin a word, then would "quadweekly", "quadriweekly", or some other word be more etymologically appropriate? For example, is there a similar series from which I could borrow the prefix for "four"?
UPDATE: "Monthly" isn't appropriate because one month is a few days more than four weeks.