For words that have only three letters, there is not any 4th letter. For instance with the word “add”, there aren't any additional letters afterwards and so has to be the word "increase". The word “addict” is while in the BIP39 wordlist however , you would've utilized “addi” for “addict”. https://iowa-bookmarks.com/story14452249/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase