For words that have only three letters, there is absolutely no 4th letter. For example with the word “incorporate”, there isn't any additional letters afterwards and so needs to be the word "insert". The word “addict” is from the BIP39 wordlist however, you would've applied “addi” for “addict”. This additional https://bookmark-media.com/story19073222/new-step-by-step-map-for-ledger-seed-phrase-bip39