For words that have only 3 letters, there isn't any 4th letter. For example with the word “increase”, there aren't any much more letters afterwards and therefore has to be the word "add". The word “addict” is from the BIP39 wordlist however , you would have applied “addi” for “addict”. https://bookmarkwuzz.com/story18796978/a-secret-weapon-for-bip39-words-to-numbers