For words that only have three letters, there's no 4th letter. As an example with the word “add”, there are no additional letters afterwards and thus have to be the word "add". The word “addict” is from the BIP39 wordlist but you would have utilised “addi” for “addict”. BIP39 and https://socials360.com/story10109450/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator