For words that only have 3 letters, there isn't any 4th letter. By way of example with the word “insert”, there aren't any more letters Later on and therefore must be the word "insert". The word “addict” is in the BIP39 wordlist however, you would have utilised “addi” for “addict”. https://linkingbookmark.com/story18848072/examine-this-report-on-bip39-mnemonic-phrase-tool