For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “insert”, there aren't any more letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist however, you would have utilised “addi” for “addict”. BIP39 https://tetrabookmarks.com/story18935714/the-5-second-trick-for-ledger-seed-phrase-bip39