For words that only have 3 letters, there is absolutely no 4th letter. By way of example with the word “insert”, there are no more letters afterwards and thus should be the word "add". The word “addict” is in the BIP39 wordlist however you would've utilised “addi” for “addict”. By https://mysterybookmarks.com/story18998884/the-5-second-trick-for-ledger-seed-phrase-bip39