For words that have only 3 letters, there isn't any 4th letter. Such as with the word “increase”, there are no extra letters afterwards and so has to be the word "include". The word “addict” is in the BIP39 wordlist but you might have applied “addi” for “addict”. Art is https://web.archive.org/web/20241213201435/https://bip39-phrase.com/