I know this is strange but I hope someone has a similar problem and better, a solution so I can use.
I have two fire tv 4k sticks, one 4k pendant, one regular fire tv stick (2nd gen). In my basement, I have an Optoma HD29 projector connected to Vizio SB36512 soundbar for a very simple home "theater" setup. My plan was to use fire tv 4k stick because the form factor allows it to hide behind the Vizio soundbar for a clean look, and the general speediness of the 4k stick over a regular 2nd gen stick. Here is what I found:
1) If I connect 4k stick to HDMI in of soundbar, then connect soundbar to projector using a long HDMI cable, I cannot turn the projector on using its manufacture remote (turn off is fine, but that's a separate button). I can, however, switch the projector on manually by pressing the power button on the projector. Considering the projector is ceiling mounted, I become the only person who can turn it on (no one else in the family is tall enough, unfortunately). Also I have to adjust the focus constantly because every time I press the power button there is a slight movement of the projector. I have tried both 4k sticks and both show the exam same symptom. The other annoying thing is that more than often when projector does power on, there is no sound in the soundbar. The only solution is to unplug soundbar and re-plug it into the outlet.
2) If I change the 4k stick to 4k pendant and use the same setup as above, projector can be switched on but 4k pendant seems to be dead. The only way to resurrect it is to unplug the USB power cable and re-plug it. It's annoying but relative do-able. However, the form factor of 4k pendant does not allow it to be hidden behind the soundbar so this solution is not optimal.
3) If I use 2nd gen stick in the place of the 4k stick, everything seems to be fine. Projector can be switched on and off normally using the manufacture remote and sound comes readily from the soundbar as expected. The YouTube app looks weird though, but that's a minor inconvenience I'm willing to live with, as well as the noticeably slower speed.
Here is my question. I thought 2nd gen stick and 4k stick are based on the similar hardware, and should have similar capabilities. Right now I firmly believe these are two different product lines that just "look like" each other. I believe 4k stick is an update from the 4k pendant (aka Fire TV 3rd gen), and 2nd gen stick is an update from the original fire tv stick. Besides what I observed from above, my 2nd gen stick also broadcast a hidden WiFi ssid at the exact same channel of my WiFi router. You can imaging my surprise when I first realized it and how it almost destroyed my WiFi network. I even exchanged the 2nd gen stick with Amazon because of that, and the replacement has the exact same behavior. The solution (which is not much a solution, more like a compromise) is to create a dedicate 5gHz ssid on my router and connect 2nd gen stick to 5gHz signal. Because 5gHz is much more tolerant of interference, it was much less of an issue.