For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “include”, there are no much more letters afterwards and so needs to be the word "include". The word “addict” is in the BIP39 wordlist however you might have made use of “addi” https://digibookmarks.com/story18951791/the-ultimate-guide-to-bip39-seed-phrase-github