For words that only have 3 letters, there's no 4th letter. One example is with the word “insert”, there won't be any extra letters afterwards and therefore have to be the word "insert". The word “addict” is while in the BIP39 wordlist but you might have employed “addi” for “addict”. https://scrapbookmarket.com/story18807844/bip39-phrase-generator-can-be-fun-for-anyone