For words that have only 3 letters, there's no 4th letter. As an example with the word “increase”, there isn't any additional letters Later on and therefore has to be the word "add". The word “addict” is from the BIP39 wordlist but you might have utilized “addi” for “addict”. One https://socialmediainuk.com/story20244227/5-essential-elements-for-bip39-explained