4.5 should be a pretty simple achievement as you have already been running at that speed. 1st off we need to lower the voltage to see how low it will go while staying stable.
Go for around 1.25v on Vcore and set your multiplier at 45. If it works and is stable while running something like occt, then reduce the voltage slowly until you find out where it becomes unstable. Alternatively, increase voltage if unstable.
You get the idea. Overclocking really is as simple as finding the right combination of multiplier and voltage to get stable. Once that has been found you are able to fettle other voltages and settings to try and eek out any more power.
Personally I would start on lowering your VCore, once that is done let me know how low it goes, and we can look at getting a little more speed depending on temps.