Is four hours of on-screen time really the standard? If that's the case then yes, I have completely unrealistic expectations. But I'll admit to having read multiple reviews of this device prior to purchasing it. Although in all honesty, the phone could have one hour of up time and I would probably still keep it because it's gorgeous. But take a look at the article below where The Verge states that the phone operated for nine hours on a Web loop, which I would guess keeps the screen up:
Samsung Galaxy S6 review | The Verge
And this site that references multiple hours of video:
Samsung Galaxy S6 and S6 Edge battery endurance revealed in tests
If that's the case, is in my four then below average or am I misreading the reviews?
Those numbers that review sites post are isolated tests. There is no standby, push services, gps, and all the other little things you use during the day. They are just running that test and if you turned off everything on your device and ran the test, then yes you'd get the same. They are the only real way of comparing devices against each other. They are useful for comparison purposes, but not really that useful for measuring real life usage. Here is one site that I really like and trust that does thorough battery tests:
Battery life tests - GSMArena.com
Again, don't take the number you see as a number that should correlate directly to your own usage. There are many things that come into play when affecting battery life that those tests can't predict, such as your type of usage, cell signal, etc. Where those review sites and ratings are beneficial are when comparing each other against other phones.
I personally think under 4 hours screen on time is bad and would be bad enough for me to return the phone. Most phones that people are using now probably get under 4, but for a brand new flagship, it would be unacceptable to me. Over 5 would be great battery life and there aren't too many phones out there now that can last that long. I do think the S6 with some optimization and possibly some updates will be able to hover close to the 5 hour mark though. Your stats reflect that, being around 4.5 hours. The fact that you're not making it to the end of the day is because of how much you are using it. It's probably because it's still new and your playing around with it. If you used it an hour less, your battery would have been 20% higher when you took that screenshot and you probably would have been happy with your battery life.
I don't know what device your coming from, but I can almost guarantee that this phone gets better battery life than your old one even if it doesn't feel like it now. Once you settle back in to your normal schedule and aren't tinkering with the phone constantly, you will see.