Don't go as low as possible on PLL, go for 'stable as possible with lowest core volts possible'
PLL voltage is just a trick to get low core volts and still remain stable.
So you will need quite some time to figure out your settings...
Give your self some time to think and work it through.
Best if you go like this:
Lower core volts @ 4.5GHz with 1.7v PLL and get to an unstable point.
Then just lower PLL by 1 or 2 - and stress again to see if it works... if not, lower PLL again, till you get it stable for at least 3h. Now lower vCore again and test. ~repeat
When you reach the lowest PLL possible, and thrust me, you will find it instantly, it will crash in a few minutes, raise it and you have your lowest core/PLL possible.
Have fun!