For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “increase”, there isn't any a lot more letters Later on and therefore have to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would have used https://tinybookmarks.com/story18938174/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool