For words that only have three letters, there is no 4th letter. Such as with the word “add”, there isn't any a lot more letters Later on and therefore must be the word "insert". The word “addict” is within the BIP39 wordlist but you would have utilised “addi” for “addict”. https://atozbookmark.com/story18832534/fascination-about-bip39-english-words