An i7 4790K runs at a 4.0GHz base clock, where the i5 4670K runs at a 3.4GHz base clock, and also has Hyperthreading , all of which increases core voltage, power dissipation and operating temperatures. If you're overclocked, you haven't mentioned what that clock speed is.
You said your i5 4670K never got above 50C "under load" ... what load? Unless you ran P95 version 26.6 Small FFT's at the same ambient, clock speed and core voltage on both processors, then you have no way to compare apples to apples.
Also, your ambient is about 25C. That's about 3C above Standard Ambient, which is 22C. Intel's thermal Specifications are based on Standard Ambient 22C. Further, at 25C ambient, 70C with Small FFT's is not hot. If we subtract the 3C you are above ambient to correct your Core temperatures to Intel's standards, then your results look even better at 67, 67, 62, 55.
Intel's Thermal Specification for your i7 4790K (Tcase) is 74C, which is CPU temperature, not Core temperature. Core temperature is 5C higher than CPU temperature due to sensor location. The corresponding Core temperature is 79C. That's the spec.
Regardless, you still have a consistent 12C spread. It appears that your particular 4790K sample has a DTS sensor that's a little out of spec on the 4th core, as neon neophyte has pointed out. Although a 12C spread isn't quite normal, it's not a problem as long as it's less than the other Cores.
77F = 25C. Here's the temperature conversions and a short scale:
Cx9/5+32=F ... or ... F-32/9x5=C ... or more simply ... an increase of 1C = an increase of 1.8F
30.0C = 86.0F Hot
29.0C = 84.2F
28.0C = 82.4F
27.0C = 80.6F
26.0C = 78.8F Warm
25.0C = 77.0F
24.0C = 75.2F
23.0C = 73.4F
22.0C = 71.6F Standard ... or ... 22.2C = 72.0F
21.0C = 69.8F
20.0C = 68.0F Cool