gust
TNT
A four letter word meaning not kosher is: tref
kill
into
aver
The word pine can have that meaning.
"four-letter word" is another phrase meaning "curse word, so the phrase might mean that boobies is not a scientific word for breasts, but it's not considered vulgar, either.
A four-letter word for cooking fat is "lard".
A four letter word meaning to grow light as the sun rises is dawn.
stop or halt is a four letter word meaning the opposite of go.
A four letter word meaning the foundation of something is base.
Writ