For words that only have 3 letters, there is no 4th letter. As an example with the word “insert”, there are no a lot more letters afterwards and therefore needs to be the word "incorporate". The word “addict” is in the BIP39 wordlist however , you would have utilized “addi” https://onelifesocial.com/story4153219/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator