For words that only have 3 letters, there is no 4th letter. As an example with the word “include”, there are no much more letters Later on and thus has to be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would've utilized “addi” for “addict”. https://bookmarking1.com/story18850992/invalid-bip39-phrase-things-to-know-before-you-buy