For words that have only 3 letters, there is not any 4th letter. By way of example with the word “increase”, there won't be any much more letters Later on and so has to be the word "insert". The word “addict” is while in the BIP39 wordlist however you might https://olivebookmarks.com/story18891351/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator