For words that only have three letters, there is absolutely no 4th letter. One example is with the word “increase”, there won't be any additional letters Later on and therefore should be the word "add". The word “addict” is from the BIP39 wordlist however, you would've employed “addi” for “addict”. https://keybookmarks.com/story18841582/new-step-by-step-map-for-bip39-english-words