For words that only have three letters, there is absolutely no 4th letter. One example is with the word “insert”, there isn't any a lot more letters Later on and therefore have to be the word "add". The word “addict” is during the BIP39 wordlist however you would've utilised “addi” https://allyourbookmarks.com/story18960481/the-invalid-bip39-phrase-diaries