For words that have only three letters, there's no 4th letter. By way of example with the word “increase”, there won't be any extra letters Later on and therefore should be the word "increase". The word “addict” is in the BIP39 wordlist however you might have made use of “addi” https://socialwebleads.com/story4240892/not-known-facts-about-bip39-english-words