4 is nothing to worry about. my house is 1 or 2. My sisters house was a 30+ before they had it better ventilated.
Radon comes from decaying, or breaking down uranium. some areas have more than others. it's in very small trace ammounts, but it's there.
What made the levels in their basement go up was that my brother in law, who was trying to save money, sealed up every nook and crack in his basement. They had inspectors come check it out and even told him that was the problem. So he just took most of the sealant away from the window edges and some other places.
His parents had a professional come in, dig a hole under their basement floor, install a pipe that went from the basement, up through "either into their attick, or through the roof." I am pretty shure it was just routed into their attick since it is well ventilated.
That was fairly cheep he said.
Don't be fooled, it isn't that big of a deal to take care of. It is a big deal, especially if you have kids, but isn't hard to fix.
Whiskey