For words that have only 3 letters, there is absolutely no 4th letter. For example with the word “add”, there isn't any much more letters Later on and therefore should be the word "increase". The word “addict” is from the BIP39 wordlist but you might have utilized “addi” for “addict”. https://livebookmarking.com/story18930194/the-basic-principles-of-bip39-passphrase-generator