For words that have only three letters, there is no 4th letter. By way of example with the word “increase”, there aren't any additional letters Later on and therefore has to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would have made use of https://mirrorbookmarks.com/story18921330/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator