For words that only have three letters, there's no 4th letter. As an example with the word “increase”, there won't be any much more letters Later on and thus need to be the word "include". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. https://binksites.com/story8506769/the-invalid-bip39-phrase-diaries