For words that have only 3 letters, there isn't a 4th letter. One example is with the word “increase”, there aren't any a lot more letters Later on and thus have to be the word "increase". The word “addict” is in the BIP39 wordlist but you would have applied “addi” https://bookmarklinkz.com/story18745808/the-ultimate-guide-to-bip39-seed-phrase-github