For words that only have 3 letters, there is absolutely no 4th letter. For instance with the word “add”, there isn't any additional letters afterwards and so needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would have applied “addi” for “addict”. For https://hubwebsites.com/story20122552/bip39-phrase-generator-can-be-fun-for-anyone