Hi Cess,
As I mentioned earlier you should be OK going to 1.5V and yes, it might help you get over the 3.9 threshold.
You really shouldn't need to unlock the cores on the CPU, unless you think there is a problem with one of them and want to disable it. You should be able to get the 1090T firing on all 6 cores though, so I wouldn't do this at such an early stage of overclocking.
Up until now, if you have followed the steps that I have suggested, all of the main settings in BIOS are still on AUTO, so a couple of things to think about as you start to fine tune other settings. The more voltage you pass through the various components the hotter things become. Also, the more overclocking you do to a component the shorter its life can become. I know that you understand this already, but it is always worth repeating.
If you are lucky, increasing the voltage to the CPU will get your CPU further up towards your target but it's important to note that other items can also have a bearing on attaining the heady heights of 4.0+ GHz. This is where it becomes harder to offer firm advice, as each motherboard/CPU/RAM combination will need slightly different settings. For example, on my GA-790XTA-UD4 Mobo with the same model CPU and 8 Gb of RAM I got to 4.2 GHz without having to make any major adjustments to voltages or timings, you can't get past 3.8GHz before you're having to make these.
However, if the voltage increase doesn't improve stability and the system still locks then you will need to start looking at possibly increasing the CPU NB VID Control also the CPU Northbridge Freq. You only need to make fine adjustments for them to have a big impact so don't be tempted to whack things up. Also make sure that you keep a very close eye on your RAM Speed if you make adjustments to the CPU North Bridge freq, as this will increase the speed of your RAM. I know your RAM is rated at 1600 MHz so I think you should be able to let it go up towards the 1500 MHz before having any real concerns but, it will be a bit of a balancing act to get everything fine tuned and running happily.