So many great places to stay within that radius. Start with Estes Park and Rocky Mountain National Park. Cheyenne Mountain State Park near Colorado Springs, but reservations are a must and hard to come by. We just had to put together 2 days on our upcoming trip, but even that is worth it. We are also including Mueller State Park in the same general area.
The Area around Buena Vista (between Leadville and Salida) in the Arkansas Valley has many good commercial campgrounds. We will be spending the Fourth of July there.
From there we move on down to Vallecito Resavoir area at Blue Spruce RV Park. That doesn't fit your "four hours from Denver" but it is where we are going.