Is mining bad for the CPU?
Its mostly the heat...if it is running quite hot, which most laptops tend to do, it IS damaging the CPU. Try a good cooling mat if you really want to do it.
Also, using 2 of 4 cores WILL run cooler than using all 4. But I guess I could repeat everyone else' advice: "Mining is no longer profitable on the CPU, don't do it." (Even though I do a little bit.)