For words that only have three letters, there is absolutely no 4th letter. As an example with the word “increase”, there isn't any far more letters Later on and therefore needs to be the word "insert". The word “addict” is within the BIP39 wordlist however, you would have utilised “addi” https://throbsocial.com/story21545809/a-review-of-bip39-passphrase