For words that only have three letters, there's no 4th letter. For instance with the word “increase”, there isn't any additional letters afterwards and therefore has to be the word "increase". The word “addict” is while in the BIP39 wordlist however , you would've used “addi” for “addict”. You could https://bookmarkick.com/story18840587/the-definitive-guide-to-bip39-seed-phrase