For words that only have three letters, there is absolutely no 4th letter. For example with the word “increase”, there aren't any more letters afterwards and therefore must be the word "insert". The word “addict” is within the BIP39 wordlist but you would have applied “addi” for “addict”. A different https://geniusbookmarks.com/story18859131/5-simple-techniques-for-bip39-word-list-pdf