For words that have only three letters, there is absolutely no 4th letter. For instance with the word “increase”, there aren't any more letters afterwards and thus needs to be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would have used “addi” for “addict”. Meticulously https://minibookmarking.com/story18919445/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition