For words that only have 3 letters, there is not any 4th letter. Such as with the word “insert”, there won't be any more letters afterwards and thus should be the word "add". The word “addict” is in the BIP39 wordlist however you would've utilised “addi” for “addict”. Mnemonic words https://altbookmark.com/story20666558/the-2-minute-rule-for-bip39-all-words