For words that only have 3 letters, there is no 4th letter. For example with the word “insert”, there aren't any far more letters afterwards and so need to be the word "increase". The word “addict” is in the BIP39 wordlist however, you would have utilized “addi” for “addict”. Getting https://getidealist.com/story20914158/bip39-seed-phrase-mnemonics-generator-an-overview