For words that only have three letters, there isn't a 4th letter. By way of example with the word “add”, there won't be any much more letters afterwards and so should be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have applied “addi” https://www.mediafire.com/file/qad32ryob76naol/pdf-7033-8222.pdf/file