Quadrimonthly / Tetramonthly / Triannual

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