For words that have only 3 letters, there is no 4th letter. One example is with the word “incorporate”, there won't be any a lot more letters afterwards and therefore must be the word "insert". The word “addict” is while in the BIP39 wordlist however you would have used “addi” https://bookmarks-hit.com/story19648369/new-step-by-step-map-for-ledger-seed-phrase-bip39