For words that only have three letters, there is not any 4th letter. For example with the word “include”, there are no extra letters afterwards and so should be the word "include". The word “addict” is inside the BIP39 wordlist but you would've utilised “addi” for “addict”. On this page https://bookmarkick.com/story18892426/an-unbiased-view-of-bip39-seed-phrase-github