For words that only have 3 letters, there isn't any 4th letter. As an example with the word “add”, there are no extra letters afterwards and therefore must be the word "insert". The word “addict” is while in the BIP39 wordlist however, you would have employed “addi” for “addict”. - https://businessbookmark.com/story4241574/new-step-by-step-map-for-ledger-seed-phrase-bip39