For words that have only three letters, there's no 4th letter. One example is with the word “increase”, there are no additional letters Later on and thus should be the word "include". The word “addict” is while in the BIP39 wordlist but you would've made use of “addi” for “addict”. https://ariabookmarks.com/story5345467/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool