Spokes are generally the toughest part of the wheel. If this was my bike, I would just replace the 4 damaged spokes. Replace them one at a time, making sure to seat the elbows before threading into the nipple. By doing them one at a time you can simply re-tension them by bringing them into true. Use a light oil like Triflow on the threads. As a shop mechanic, I did this exact job countless times without any problems down the road.

Your 9 speed cassette didn't cause this to happen. You didn't adjust your low limit stop on the RD, which should be checked anytime you install a new cassette or wheel.