There's monthly, bimonthly and quarterly.. So what's the word to indicate "every four months"? Apparently, nobody knows.. but the choices were Tetramonthly (which sounds horrible), triannual (which indicates three years rather than a third of a year.. i.e, it's the same as triennial) and lastly, quadrimonthly, which, atleast to yours truly, makes the most sense. Moreover, if Big Blue can use it, so can Quillem :)
-K