For words that have only three letters, there is absolutely no 4th letter. For example with the word “add”, there aren't any additional letters afterwards and so need to be the word "insert". The word “addict” is while in the BIP39 wordlist however, you might have utilized “addi” for “addict”. https://socialtechnet.com/story4500838/the-invalid-bip39-phrase-diaries