What do lengths of time with the bi prefix mean? I have understood bicentennial as once every two hundred years, but biannual as meaning twice a year. Do biweekly and bimonthly mean twice a week

My guess it there might be one that i don't know of. 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 approp