For words that have only 3 letters, there is no 4th letter. By way of example with the word “incorporate”, there aren't any extra letters Later on and so need to be the word "incorporate". The word “addict” is in the BIP39 wordlist however you would have utilized “addi” for https://listbell.com/story9388205/ledger-seed-phrase-bip39-no-further-a-mystery