For words that have only 3 letters, there isn't any 4th letter. For example with the word “add”, there isn't any far more letters Later on and therefore has to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you might have utilised “addi” for “addict”. https://yourbookmarklist.com/story19033993/ledger-seed-phrase-bip39-no-further-a-mystery