For words that have only 3 letters, there isn't a 4th letter. As an example with the word “increase”, there won't be any additional letters Later on and so has to be the word "increase". The word “addict” is within the BIP39 wordlist however , you might have used “addi” https://gorillasocialwork.com/story20397760/a-secret-weapon-for-bip39-words-to-numbers