For words that only have three letters, there's no 4th letter. For instance with the word “incorporate”, there are no additional letters afterwards and so must be the word "insert". The word “addict” is while in the BIP39 wordlist but you might have made use of “addi” for “addict”. This https://dirstop.com/story21881650/getting-my-bip39-words-to-numbers-to-work