For words that have only 3 letters, there's no 4th letter. As an example with the word “increase”, there aren't any more letters Later on and therefore have to be the word "increase". The word “addict” is from the BIP39 wordlist however , you would have made use of “addi” https://kbookmarking.com/story19014528/the-greatest-guide-to-seed-phrase-bip39