For words that only have 3 letters, there's no 4th letter. One example is with the word “increase”, there aren't any a lot more letters afterwards and thus has to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would have used “addi” https://socialdosa.com/story9336219/bip39-phrase-generator-can-be-fun-for-anyone