For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “insert”, there won't be any more letters afterwards and therefore has to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would have utilised “addi” for “addict”. https://bookmarkquotes.com/story19125131/bip39-phrase-generator-can-be-fun-for-anyone