For words that only have 3 letters, there isn't any 4th letter. As an example with the word “increase”, there isn't any a lot more letters afterwards and so has to be the word "add". The word “addict” is while in the BIP39 wordlist however you might have made use https://dftsocial.com/story19826443/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator