For words that have only 3 letters, there's no 4th letter. For instance with the word “add”, there isn't any a lot more letters afterwards and so needs to be the word "include". The word “addict” is within the BIP39 wordlist however, you would've applied “addi” for “addict”. A seed https://thesocialcircles.com/story4708851/bip39-seed-phrase-mnemonics-generator-an-overview