For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “increase”, there are no much more letters Later on and therefore needs to be the word "insert". The word “addict” is within the BIP39 wordlist but you would've used “addi” for https://getsocialsource.com/story4196883/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool