For words that only have 3 letters, there isn't any 4th letter. By way of example with the word “add”, there are no extra letters Later on and therefore have to be the word "increase". The word “addict” is from the BIP39 wordlist but you would've applied “addi” for “addict”. https://mnobookmarks.com/story18960672/bip39-seed-phrase-fundamentals-explained