I've watched some of Amazon's 4K content and wasn't particularly blown away. In fact, I had to pause the Fire TV a few times just to see whether or not the stream was still in 4K, because if your network traffic slows down it will revert back to 1080. It wasn't noticeable enough for me on a 50" screen at about 8 feet away.
Have you verified that the HDMI port you're using supports 4K/the nessesary HDCP? I stream netflix directly on my smart TV (as well as youtune 4k), and like I said, the difference is immediately obvious. I suspect you're getting 4k downscaled to 1080p.
It's definitely working -- in fact, the FireTV won't even display the "4K Ultra HD" categories unless it's connected correctly to a compatible TV[0]. I notice the difference if I actively try to, I just don't think it's as mind blowing as everyone is claiming it to be. It's certainly not something I couldn't live without.
