For words that only have 3 letters, there is not any 4th letter. For instance with the word “increase”, there are no far more letters afterwards and thus has to be the word "include". The word “addict” is in the BIP39 wordlist but you might have utilized “addi” for “addict”. https://mysitesname.com/story8561903/facts-about-bip39-seed-phrase-github-revealed