For words that have only 3 letters, there is not any 4th letter. One example is with the word “increase”, there won't be any a lot more letters afterwards and thus must be the word "increase". The word “addict” is in the BIP39 wordlist however , you would have applied https://bookmarkuse.com/story18632820/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting