The phrase 'CRAZY as a ___' is a popular idiom used to describe someone who is acting in an irrational or unpredictable manner. The four crossword solutions for this phrase are LOON, LOCO, GAGA, and ZANY, all of which are four-letter words. LOON refers to a type of aquatic bird, while LOCO is derived from the Spanish word for 'CRAZY'. GAGA is a slang term for someone who is eccentric or wild, and ZANY is an adjective meaning 'clownish or comical'. All of these words can be used to describe someone who is acting in an unpredictable or irrational manner.