For words that only have 3 letters, there isn't any 4th letter. As an example with the word “include”, there are no more letters Later on and so should be the word "insert". The word “addict” is inside the BIP39 wordlist but you would have utilised “addi” for “addict”. Standard https://socialbuzzmaster.com/story4615133/a-review-of-bip39-phrase-list