Thanks everyone for the input.
We're certainly going to forego lending to them, and I told them already that the money in tied up in stocks and making more than 4%, they understand. I think at first I was just shocked by the immediate and fierce"no" I received from the wife and took it as she had something against my parents. My initial post was more of a general question if this is considered an off limits move, but it seems like it's really more specific on the details.
I certainly assume and hope the money in the stock market is making way more than 4% on average. As for my parents, yes they can take out a HELOC which is the plan and the reason they offered us ~4% return, but they made the offer with the assumption that our house money was not tied up in the stock market, and was sitting safely in the bank.
Had the stock market been volatile and we cashed out of the stock market and had the cash in the bank, I'd say the 4% isn't a bad offer.