For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there are no more letters Later on and therefore has to be the word "include". The word “addict” is in the BIP39 wordlist however , you would have used “addi” for “addict”. https://bookmarkinglife.com/story4568462/facts-about-bip39-seed-phrase-word-list-revealed