For words that have only three letters, there's no 4th letter. For example with the word “insert”, there aren't any extra letters Later on and so have to be the word "include". The word “addict” is in the BIP39 wordlist but you would have made use of “addi” for “addict”. https://reallivesocial.com/story4396785/bip39-seed-phrase-mnemonics-generator-an-overview