quattuor

Quattuor is indeclinable, which makes it easy: it never changes, whatever case the noun beside it takes. Only unus, duo and tres decline; from quattuor to centum the cardinals sit still.

The rest of the family does move. The ordinal is quartus, "fourth", a normal adjective; the distributive is quaterni, "four each"; the adverb is quater, "four times". Medieval scribes very often write quatuor with a single t, and the corpus shows both spellings, so do not treat the shorter one as an error. The root is behind English quart, quarter, quarantine (forty days, via quaranta), square and squadron, all through the compounding form quadr-.