The temps are all perfectly safe, and it's not unusual for one core to be hotter than another. A lot of programs only use one core, and if one core is getting heavier use, it'll generate more heat. Or, it could be that the thermal paste came out a little unevenly.
If these are just idle temps, you might try running a benchmark that maxes out all four cores at the same usage. If it's still giving greatly different results, you can reapply the thermal past if you want. Although it's also possible that there's nothing to be done about it. Just like no two CPUs in the same batch are exactly the same, one core can just be a little different from another and run hotter.