For words that have only three letters, there is absolutely no 4th letter. For example with the word “add”, there aren't any a lot more letters afterwards and thus must be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've utilised “addi” for “addict”. For https://maximusbookmarks.com/story19129834/the-ultimate-guide-to-bip39-seed-phrase-github