For words that only have 3 letters, there's no 4th letter. As an example with the word “include”, there are no far more letters Later on and therefore has to be the word "increase". The word “addict” is during the BIP39 wordlist but you would've used “addi” for “addict”. On https://wise-social.com/story4512511/the-ultimate-guide-to-bip39-seed-phrase-github