Intel Core i7 cores have different temps
Should I be concern about the 7-8 degree difference between core 0 and core 3?
Some difference is normal, since the mating surface to the HSF is never really perfect and because different cores may be presented with different loads. My own system is 2~5 degress different between cores. So yours isn't all that out of line.
At most, you may want to remove the heat sink/fan, clean the respective surfaces, reapply TIM, and re-mount. But as long as your overall temps are under control, I don't feel it's necessary. But if you're like me, this kind of stuff tends to sit in the back of your head and fester. So... certainly wouldn't hurt.