I've had my car up on four axle stands a couple of times with no issues. As others have mentioned it's important to keep the car level, which may mean using different settings on your stands depending on where you place them under the car (the rear axle sits at a different height relative to the ground than the front cross member).
If you're doing this on asphalt or really anything other than concrete you will want to place sheets of thick plywood under the stands so that they do not sink into the ground, which could cause them to tip over.
The other useful lesson I learned was that you have to jack from the rear first, then from the front. If you lift the front first there will not be enough clearance to get the jack under the rear of the car.
