For words that only have 3 letters, there is not any 4th letter. For example with the word “include”, there aren't any additional letters afterwards and so must be the word "add". The word “addict” is in the BIP39 wordlist but you would've applied “addi” for “addict”. Mnemonic words examples https://checkbookmarks.com/story4387018/not-known-factual-statements-about-bip39-explained