The unit for the psychiatric ward at Tripler Army Medical Center, in Honolulu Hawaii.
The term "4b2" is used in context for someone sent to the psych ward for mental breakdown, or more commonly, for alcohol detox. It is the military equivalent of 51-50 in Hawaii.
"They 4b2'ed him after he ran around base drunk."
"He's a 4b2'er, watch out."
"He wasn't well after he got back, so they sent him to 4b2."
"He wasn't wearing any chevron's, so we knew he had just come from 4b2."
"He's a fellow 4b2'er, we're all nuts man."