For words that have only three letters, there is absolutely no 4th letter. For instance with the word “increase”, there isn't any a lot more letters afterwards and so should be the word "add". The word “addict” is from the BIP39 wordlist however you would've used “addi” for “addict”. Mnemonic https://bookmarkstown.com/story19023238/not-known-facts-about-bip39-english-words