For words that only have three letters, there is no 4th letter. For instance with the word “add”, there isn't any more letters afterwards and thus need to be the word "increase". The word “addict” is from the BIP39 wordlist but you would have utilized “addi” for “addict”. Mnemonic words https://bookmarkstown.com/story19088513/an-unbiased-view-of-bip39-seed-phrase-github