For words that only have 3 letters, there's no 4th letter. As an example with the word “insert”, there isn't any far more letters afterwards and so has to be the word "add". The word “addict” is within the BIP39 wordlist however you would have employed “addi” for “addict”. You https://sites2000.com/story8457131/ledger-seed-phrase-bip39-no-further-a-mystery