For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “insert”, there won't be any more letters Later on and therefore needs to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however , you would have used https://madesocials.com/story4295709/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting