For words that have only three letters, there isn't a 4th letter. One example is with the word “insert”, there won't be any more letters afterwards and thus must be the word "increase". The word “addict” is during the BIP39 wordlist however you would have applied “addi” for “addict”. This https://dirstop.com/story22266167/little-known-facts-about-bip39-seed-phrase-list