For words that only have three letters, there is no 4th letter. For example with the word “insert”, there are no a lot more letters afterwards and so needs to be the word "increase". The word “addict” is from the BIP39 wordlist however you would've made use of “addi” for https://bookmarkboom.com/story18838997/the-definitive-guide-to-bip39-seed-phrase