Amazon currently only has it in HD (although that may change soon as many of my Universal 4K MA-eligible movies have been slowly upgraded to 4K), but iTunes (aka Apple TV) has it in 4K for $4.99.
This is the Joss Whedon Serenity, not to be confused with the Matthew McConaughey garbage Serenity (both released by Universal).
At first, I too thought the 4K was not on Amazon, but found it listed separately from their regular HD/SD listing, which seems Amazon's typical approach for 4K. Also, just (logged off my account because I already bought it on sale via iTunes, and) rechecked their 4K listing, and it's not at a competitive sale price.
_Man_