Firestick 4k's auto resolution/framerate doesn't work right for most TVs. So when you set the resolution to Auto, it defaults to 1080p 60Hz for everything other than the Prime Video app. However, you get Dolby Vision.
If you set it to 4k/60 mode, on many TVs it won't work with Dolby Vision as they don't accept 4k 60 Dolby Vision signal (also I'm not sure if firestick 4k can even support 4k 60 output in DV, as Apple advertised a lot that their new Apple TV 4k can support 4k 60 DV).
You can switch to 4k 24, 4k 25 or 4k 30 through a hidden option, but then the interface becomes laggy and you lose the ability to play 60fps videos on youtube.
I ended up selecting 4k/60 forced and HDR to auto, with bit-depth as up to 12-bits. This will give you HDR10 in HDR10 and DV titles, and HDR10+ in 10+ titles if your TV supports it, else HDR10.