For words that have only three letters, there's no 4th letter. As an example with the word “add”, there won't be any more letters afterwards and thus need to be the word "incorporate". The word “addict” is while in the BIP39 wordlist but you might have employed “addi” for “addict”. https://bookmarkingdepot.com/story18762317/fascination-about-invalid-bip39-phrase