STUD !
Boy
sonny
HUNK?
lad
Lad
post
Got to be "Door."
young man
Boy or lad or kid would fit.
The four-letter word that can go before "step," "bell," and "amp" is "hand." Together, they form "handstep," "handbell," and "handamp."
Boy Guy and Lad are also two of the probable answers.
Got to be "Door."