According to https://en.wikipedia.org/wiki/Numeral_prefix, I would say, it should be trientannually. I googled this word after creating it but couldn't find it on Google but I really think it should be in google. So here I am contributing. Please feel free to let me know if this is incorrect. thanks!
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According to the online Oxford Dictionaries, "bimonthly" can refer to something occurring every two months or twice in one month. It might be best to just say "every two months" to avoid confusion.
From the Related Link below: ; bimonthly / semimonthly : Bimonthly is an adjective that means every two months: "I brought the cake for the bimonthly office party." Bimonthlyis also a noun that means a publication issued every two months: "The company publishes several popular bimonthlies." Semimonthly is an adjective that means happening twice a month: "We have semimonthly meetings on the 1st and the 15th."
The adjective is quadrennial. (extension of 1-annual, 2-biennial, 3-triennial) (The archaic word for a five-year period is a lustrum.)
It depends on what you mean by 'four letter word'. Thea has four letters, so yes it is. It is not a swear word, so it's not that kind of 'four letter word'.