For words that only have 3 letters, there is absolutely no 4th letter. For instance with the word “add”, there isn't any extra letters Later on and so have to be the word "increase". The word “addict” is during the BIP39 wordlist however, you would have made use of “addi” https://thesocialvibes.com/story4517051/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator