For words that only have 3 letters, there is no 4th letter. One example is with the word “include”, there isn't any additional letters Later on and so must be the word "insert". The word “addict” is inside the BIP39 wordlist however , you might have made use of “addi” https://bookmarkspy.com/story20228621/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition