For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “insert”, there aren't any more letters afterwards and so should be the word "include". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. This extra complexity https://sound-social.com/story8921370/the-5-second-trick-for-ledger-seed-phrase-bip39