For words that only have 3 letters, there's no 4th letter. As an example with the word “incorporate”, there isn't any more letters Later on and thus have to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would've applied “addi” for “addict”. This https://isocialfans.com/story4278660/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator