For words that only have three letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there are no additional letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist but you might have utilized “addi” for “addict”. https://socialdosa.com/story8614080/the-2-minute-rule-for-bip39-all-words