For words that only have three letters, there is absolutely no 4th letter. For example with the word “add”, there won't be any additional letters afterwards and thus should be the word "add". The word “addict” is from the BIP39 wordlist however , you would've utilised “addi” for “addict”. Tips https://madesocials.com/story5092352/a-review-of-bip39-phrase-list