For words that have only three letters, there's no 4th letter. One example is with the word “insert”, there aren't any far more letters Later on and thus need to be the word "increase". The word “addict” is from the BIP39 wordlist but you might have applied “addi” for “addict”. https://bookmarksurl.com/story4276147/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator