For words that only have three letters, there is absolutely no 4th letter. For example with the word “increase”, there isn't any additional letters Later on and therefore have to be the word "increase". The word “addict” is from the BIP39 wordlist but you would have made use of “addi” https://bookmarkerz.com/story18926579/a-secret-weapon-for-bip39-words-to-numbers