For words that only have three letters, there's no 4th letter. For example with the word “incorporate”, there aren't any additional letters Later on and therefore has to be the word "add". The word “addict” is from the BIP39 wordlist but you would've employed “addi” for “addict”. By following the https://adsbookmark.com/story19664356/ledger-seed-phrase-bip39-no-further-a-mystery