For words that only have three letters, there's no 4th letter. As an example with the word “add”, there isn't any far more letters Later on and so has to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would've applied “addi” for “addict”. https://bookmarks4seo.com/story18939593/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator