For words that have only 3 letters, there is no 4th letter. For instance with the word “increase”, there aren't any additional letters afterwards and therefore has to be the word "include". The word “addict” is in the BIP39 wordlist however , you might have employed “addi” for “addict”. For https://socialfactories.com/story4319840/5-simple-techniques-for-bip39-seed-phrase