For words that have only 3 letters, there is no 4th letter. One example is with the word “insert”, there aren't any additional letters Later on and so must be the word "include". The word “addict” is inside the BIP39 wordlist but you might have utilised “addi” for “addict”. Safety: https://bbsocialclub.com/story20688216/invalid-bip39-phrase-things-to-know-before-you-buy