For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “incorporate”, there aren't any a lot more letters afterwards and thus should be the word "add". The word “addict” is in the BIP39 wordlist however you would have used “addi” https://allkindsofsocial.com/story4353850/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting