For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “increase”, there are no additional letters Later on and so need to be the word "increase". The word “addict” is from the BIP39 wordlist however you would've employed “addi” for “addict”. bip39-phrase.com https://bookmarksfocus.com/story4519621/details-fiction-and-bip39