For words that have only three letters, there isn't a 4th letter. As an example with the word “add”, there won't be any more letters afterwards and so has to be the word "increase". The word “addict” is inside the BIP39 wordlist however, you would have utilised “addi” for “addict”. https://bookmarknap.com/story9203275/ledger-seed-phrase-bip39-no-further-a-mystery