For words that have only three letters, there is no 4th letter. By way of example with the word “increase”, there aren't any extra letters afterwards and therefore has to be the word "add". The word “addict” is in the BIP39 wordlist however , you would have employed “addi” for https://bookmarkinglife.com/story4376098/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting