For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any much more letters Later on and so have to be the word "add". The word “addict” is during the BIP39 wordlist but you would've applied “addi” for “addict”. You https://zenwriting.net/keenanctdp/h1-b-unlocking-the-power-of-bip39-a-comprehensive-look-at-word-lists-and