At least two of the processors we tested required 1.44Vcore for 4.5ghz. The temperature rapidly rises as soon as you hit over 1.4V. At 1.4V, it is still very manageable but you can only clock it at 4.3ghz

4.5GHz @ 1.3V +- 0.05V is possible to pass at least 2 hours or more of Prime95. Don't take it as a 100% guarantee as different chips in different batches will demand different amount of volts to be stable. 4.5Ghz is a good range as the volts needed for 4.6GHz or more is going to be a lot, temps increase a lot as well.

91C is too high, or at least too high by my standards. 1.3V should be a sweet spot, anything less you'll risk some stability. H50 isn't enough. You should get something like a H100 or air coolers like the Noctua NH-D14 or similar.