For words that have only three letters, there is no 4th letter. By way of example with the word “insert”, there are no more letters Later on and so needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however, you would have employed “addi” https://topsocialplan.com/story5160648/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator