CPU temp 80 degrees on Start-up! and Task Manager Issue

CPU (i7 - 4790) temp 80 degrees on Start-up! the stock fan is really noisy and whenever i open the task manager and go on the performance tab where can i monitor the Cpu it suddenly decrease the temp like it's really weird? (You can hear the stock fan decrease the noise) I dont know what is the possibilities or the cause of it. But i install a Malware Bytes and Windows Security Essentials and quickly make a scan after that my PC returns to a normal state (probably a virus or worm hit me) but in a 40 degrees or sometimes 50 degrees temp. Before this issue happened i got my temp as normal of 30 degrees or sometimes it may drop to 20 or 25 and not that bad for a cpu temp.

This issue happened when i start rendering on sony vegas pro 13 and having my CPU temp rising on 80 degrees is that normal for rendering? I don't really do overclock and its fine for me to use that 7 core for my rendering projects. And the issue starts! Whenever i'm already on the startup my temp is rising on 80 without doing anything. So i figured out that i may re-add the thermal paste. It fix the problem first but this issue again comeback and i already re-add the thermal paste but it doesn't decrease.

For now im hitting 40-50 degrees temp while on idle mode or browsing internet! and when i play games or do something it rises on 60-70 (before this issue happened i only hit 50 degrees just by playing games and on ultra graphics ( so weird?)

The Software: i used is CPU Thermometer and also i would want to ask why is that my core on CPU is just only 4?

Temperature
Core #1 ----- 50
Core #2 ----- 48
Core #3 ----- 38
Core #4 ----- 40

I have 7 cores right? Where are the remaining 3 cores sorry for this stupid question but i'm a little bit confused and i want Professionals/Peoples that have knowledge on PC to explain why is that happening?

I'm sorry for this long explanation but i'm pretty worried on my CPU because i credited it on a store and still paying it monthly. I don't want to let it burn or have issues on it and probably hit the time where i can't use it.

I assume that the problem is my CPU Cooler? Hyper 212 Evo CM is probably out of stock here in my country. Can you kindly give me advice for other products with the same specification and price like CM Hyper 212 ? Or a little bit like CM 212? I have seen a Deep Cool cpu cooler having 6 pipes i think and looks like a CM 212? Is it worth it to buy?

Thank you guys for your answers i hope i may help other people having the same problem as me