For words that have only three letters, there is absolutely no 4th letter. For example with the word “add”, there isn't any extra letters afterwards and so need to be the word "increase". The word “addict” is from the BIP39 wordlist but you would have made use of “addi” for https://infopagex.com/story4341116/considerations-to-know-about-bip39-word-list-pdf