This is a very interesting thread. I had to deal with the frustration from this "unable to wake from sleep" issue since November but the 2/17/2016 update seems to have fixed it for me!
For those that encountered this issue for the first time, I feel your pain. I have lived through it for 3 months (hibernate to the rescue) and now it finally seems to be fixed for me.
UPDATE:
I have continued to test it and determined that the "unable to wake from sleep" issue is not fully resolved. In my case, it seems to be going to sleep and waking up fine as long as power supply is not involved. But with power supply (I use
Surface dock power cable), I have been able to find a couple of scenarios where it still fails to wake from sleep.
Scenario 1: If Surface Pro 4 is disconnected from power supply and I close the lid, it goes to sleep per my "close the lid" setting. While it is sleeping, I connect the magnetic power supply cable. Now when I open the lid and try to wake
it up, the IR light and keyboard light up but Surface fails to wake up from sleep. I have to close the lid again to make it go to sleep mode in power supply "connected" mode. Then it wakes up on opening the lid and pressing a key/button.
Scenario 2: If Surface Pro 4 is connected to power supply cable and you close the lid (it goes to sleep) and then you disconnect the power supply cable and then open the lid and press a key, it won't wake up from sleep. The IR light and
keyboard light up but screen stays dark. I have to close the lid again to make it go to sleep mode in power supply "disconnected" mode. Then it wakes up on opening the lid and pressing a key/button.
So the bottom line is to try to wake it up in the same power supply cable connected or disconnected state that it went to sleep in. Otherwise, close and open the lid a second (or sometimes 3rd) time.
I will post more as I test more.