Question about how to invest money that isn't for retirement

I've read and learned about investing money for the long-term into low cost index funds and letting them grow with time. But if I have money that is already invested in mutual funds and that has been growing over time and which I would like to use in 4-5 years for a down payment (for example), then how is it best to invest that money? An index fund tied only to stocks could potentially take a plunge, which is okay if you're going to let it sit until retirement, but how is best to both protect and invest money that has already been growing and which you may need sometime this decade?