For words that only have 3 letters, there is no 4th letter. Such as with the word “insert”, there won't be any additional letters afterwards and so has to be the word "increase". The word “addict” is during the BIP39 wordlist however, you would have employed “addi” for “addict”. Standard https://my-social-box.com/story4194124/bip39-phrase-generator-can-be-fun-for-anyone