For words that have only three letters, there is no 4th letter. As an example with the word “include”, there isn't any extra letters afterwards and so must be the word "insert". The word “addict” is inside the BIP39 wordlist but you might have used “addi” for “addict”. In this https://bookmarkmiracle.com/story20390609/the-2-minute-rule-for-bip39-all-words