For words that only have three letters, there's no 4th letter. Such as with the word “include”, there are no a lot more letters Later on and therefore has to be the word "add". The word “addict” is in the BIP39 wordlist however , you would've applied “addi” for “addict”. https://bookmarkextent.com/story21320686/5-simple-techniques-for-bip39-word-list-pdf