For words that have only three letters, there is not any 4th letter. By way of example with the word “add”, there isn't any far more letters Later on and so must be the word "incorporate". The word “addict” is from the BIP39 wordlist however you would have applied “addi” https://geniusbookmarks.com/story18822593/bip39-seed-phrase-mnemonics-generator-an-overview