For words that have only three letters, there is absolutely no 4th letter. For instance with the word “increase”, there aren't any a lot more letters afterwards and therefore needs to be the word "include". The word “addict” is from the BIP39 wordlist however you would've utilized “addi” for “addict”. https://funny-lists.com/story19930333/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition