For words that have only three letters, there's no 4th letter. One example is with the word “incorporate”, there aren't any more letters Later on and therefore should be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have used “addi” for “addict”. Safety: https://infopagex.com/story4183265/the-2-minute-rule-for-bip39-all-words