For words that have only three letters, there isn't any 4th letter. For instance with the word “increase”, there are no extra letters Later on and therefore need to be the word "increase". The word “addict” is within the BIP39 wordlist however , you would've used “addi” for “addict”. In https://travialist.com/story9718521/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39