For words that only have three letters, there is not any 4th letter. By way of example with the word “add”, there are no far more letters Later on and thus need to be the word "incorporate". The word “addict” is within the BIP39 wordlist but you would've employed “addi” https://wearethelist.com/story20716686/5-simple-techniques-for-bip39-word-list-pdf