For words that only have three letters, there is not any 4th letter. For example with the word “add”, there isn't any extra letters Later on and therefore has to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would have made use of “addi” https://hotbookmarkings.com/story19003004/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator