i would take the heatsink off and re apply thermal paste the apply it again to be certain
Im certainly not an expert but I'm not sure how that would help. The state of paste doesn't affect the accuracy of the sensor does it? And in my case the temp seems to constantly be lower than the other cores (in stress) so I cannot see how redoing the paste would do any good.
Or do you think that the temperature of 4th core might actually be a lot higher than the sensor shows?