Compatible TV's for IPlayer 4K are listed here uhd-connected-tv (the list doesn't contain projectors)
Have you gone into the IPlayer settings and switched to IPlayer Beta which shows the UHD content? You can play a UHD test loop from there.
Searching UHD at present shows His Dark Materials, Dracula and Dynasties which are labelled UHD. It also brings up Seven Worlds, One Planet and Blue Planet 11 which although not labelled as UHD do play in UHD.