For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “insert”, there won't be any far more letters Later on and so has to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however, you would have utilized “addi” https://highkeysocial.com/story4277350/5-essential-elements-for-bip39-explained