For words that only have three letters, there's no 4th letter. For example with the word “insert”, there aren't any more letters Later on and thus have to be the word "add". The word “addict” is from the BIP39 wordlist however you would have applied “addi” for “addict”. Mnemonic words https://bookmarkbooth.com/story18866935/little-known-facts-about-bip39-seed-phrase-list