For words that only have 3 letters, there isn't any 4th letter. By way of example with the word “include”, there are no far more letters afterwards and thus has to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would've employed “addi” for “addict”. https://socialmediastore.net/story19487093/bip39-seed-phrase-mnemonics-generator-an-overview