For words that only have 3 letters, there isn't any 4th letter. For example with the word “increase”, there isn't any additional letters afterwards and so must be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've applied “addi” for “addict”. An easy backup method https://bookmarklinking.com/story5314064/fascination-about-bip39-english-words