For words that have only three letters, there is absolutely no 4th letter. Such as with the word “insert”, there are no additional letters Later on and thus need to be the word "add". The word “addict” is inside the BIP39 wordlist but you would have made use of “addi” http://hectorswwo172.bearsfanteamshop.com/understanding-your-copyright-security-the-crucial-guide-to-bip39-passphrases-and-mnemonics