For words that have only three letters, there is absolutely no 4th letter. For example with the word “increase”, there isn't any more letters afterwards and thus needs to be the word "add". The word “addict” is inside the BIP39 wordlist however, you might have applied “addi” for “addict”. Art https://eternalbookmarks.com/story18985242/considerations-to-know-about-bip39-word-list-pdf