I was told early on that bu4 is never used alone, even in a short response to a question, though I have heard it used alone that way (but only that way) in everyday speech (admittedly, by a native Mandarin speaker who actually knows far more English than Chinese). I'm told the proper way is always to couple it with a verb, as you state.

"Do you have one?" "Ni2 yao4 yi1ge?"
"Don't have." "Bu2 yao4."

(I'm probably messing up tones.)