I pretty much agree with Xristo on what he said, in my case cores 1,2 get most of the load and are the ones that should be working at higher temps than the others. And as said it seems like your paste isnt as well spread out to help with temps on the cores 3,4
But you only uploaded the picture of temps, and it would be of great interest to have your temps together with ur 'task manager' cpu workload so that it can be seen that the cores with the most workload are the ones with the most temperature on them? (its not a matter of cores its a matter of workload on which cores)
If you do indeed re-seat the heatsink please let us know if that was indeed the problem?