For words that have only 3 letters, there's no 4th letter. As an example with the word “include”, there are no more letters afterwards and so has to be the word "add". The word “addict” is from the BIP39 wordlist however, you would've employed “addi” for “addict”. Artwork is actually https://bookmarksurl.com/story4299574/ledger-seed-phrase-bip39-no-further-a-mystery