Is the new Dashboard not in 4K resolution?

While I doubt anyone could say "they know", we can speculate on what would be plausible.

I see no reason to why the Dashboard would be a Native 4k, rather just an Upscale version of the current Dashboard. It will be essentially the same Dashboard on current Xbox Ones in the wild and the same Service of Xbox LIVE just Upscale to 4k.

Further, I see no reason why there would not be multiple resolution settings to support a multiple of output. In short, yes 480p, 720p, 1080p, 2k (maybe), and 4k upscale.

4K is about the resolution that is capable, not the power of a system. A 4K tv means its capable and will support 4K resolution. It all depends on if a game is made and supports 4K if it will display in 4K on a 4K tv.

Nope, it will honor your set resolution.

From https://support.xbox.com/en-US/xbox-one/console/adjust-display-settings

Allow 4K

This setting allows you to pass native 4K content to your display when available in your games and apps.

You don't have to set your console resolution to 4K to play 4K content. You can set your console to another resolution, such as 1080p, and when a game or app requires 4K (and your TV supports the 4K mode
it wants), it'll automatically switch to 4K. This is good if you want to play 1080p content in its native resolution, but also take advantage of the 4K resolution when playing 4K content.

It will only switch when the Game or App "Requires" 4K

If that is the case there is a bug in the software because that is the way my console is configured and the bluray app is still up scaling I have attached 1 picture of the dashboard showing resolution of 1080p and another of Ninja Turtles in regular bluray
showing a resolution of 4k not 1080p

Interesting! And the Blu-Ray is a UHD disc?

99% of consumers do not own a 4K resolution TV to experience the 4K resolution Blu-Ray's so that will not be a problem until a decade from now when 4K is the norm.