For words that only have three letters, there is no 4th letter. One example is with the word “add”, there are no a lot more letters Later on and so needs to be the word "increase". The word “addict” is during the BIP39 wordlist however, you might have utilized “addi” https://getsocialselling.com/story4215382/the-ultimate-guide-to-bip39-seed-phrase-github