For words that only have three letters, there is absolutely no 4th letter. As an example with the word “insert”, there are no much more letters Later on and therefore need to be the word "include". The word “addict” is from the BIP39 wordlist however, you would have utilized “addi” https://socialdosa.com/story8640808/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition