For words that have only 3 letters, there isn't any 4th letter. As an example with the word “add”, there won't be any much more letters Later on and thus must be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would've employed “addi” for “addict”. https://bookmarklinking.com/story4972166/bip39-seed-phrase-mnemonics-generator-an-overview