For words that have only three letters, there is not any 4th letter. As an example with the word “increase”, there isn't any additional letters Later on and therefore has to be the word "include". The word “addict” is from the BIP39 wordlist however , you might have applied “addi” https://tripsbookmarks.com/story18875956/not-known-factual-statements-about-bip39-explained