For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “increase”, there are no additional letters Later on and thus needs to be the word "increase". The word “addict” is in the BIP39 wordlist however, you would have utilised “addi” for https://doctorbookmark.com/story18855213/facts-about-bip39-seed-phrase-word-list-revealed