The four words below each have three letters and rhyme, but they also have something else in common. The same change to each word will create four new words, only two of which will rhyme.

BET
MET
PET
WET

HINTS:
#1: The change comes after the first letter of each word.
#2: The change involves three letters.

Answer:

The letters ALL can be added after the first letter of each word to create four new words:

BALLET
MALLET
PALLET
WALLET 

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
ALL ABOARD / ALL ABOARD II