When Amazon announced their add-on subscription option for Prime Video and brought Smithsonian Earth content to the Fire TV for the first time, it was a bit disapointing that the videos weren’t available in 4K UHD resolution. A week later, when a standalone Smithsonian Earth app was released, it too lacked 4K video. Thankfully, that fault has now been addressed. The Smithsonian Earth app now streams in 4K UHD on the 2nd-gen Fire TV. Big thanks to vulcan_on_earth on reddit (aka Vulcan195 in the comments here) for pointing this out. The Smithsonian Earth app requires the same $3.99 per month subscription as the add-on option for the same content, so if you have the newer Fire TV and a 4K television, it makes more sense to subscribe directly through the app, since the Prime Video add-on option still only streams in 1080p.
Smithsonian Earth is now the 3rd source of 4K content on the Fire TV, with Amazon Video and Netflix being the other two. Since the release of the 4K capable Fire TV, there certainly hasn’t been an influx of 4K apps like we would have hoped. This may be partially due to the hurdles of developing a 4K capable app, but at least the number of 4K apps has finally increased.