For words that have only three letters, there is absolutely no 4th letter. For instance with the word “increase”, there won't be any additional letters afterwards and therefore has to be the word "include". The word “addict” is from the BIP39 wordlist however, you might have used “addi” for “addict”. https://hylistings.com/story19931133/5-essential-elements-for-bip39-explained