For words that have only 3 letters, there isn't a 4th letter. One example is with the word “insert”, there won't be any far more letters afterwards and so needs to be the word "increase". The word “addict” is from the BIP39 wordlist however , you would have made use https://classifylist.com/story20626124/ledger-seed-phrase-bip39-no-further-a-mystery