Sure, under 80 is just fine. I'd really start to worry if it was over 90c consistently. I've had my CPU hit 90c during overclocking tests and there didn't seem to be any damage.
Also, try playing with the memory voltage and QPI, it's possible that you don't actually need that much voltage to the CPU and might be able to run a bit cooler by backing it down. If you've hit a sweet spot then I'd say you're doing well to hit 4.0 and less than 80c (assuming it's stable).
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