The source stream was 1080p, but LG may be onto something.
In a convergence of several prevalent tech trends, LG has been demonstrating its wireless Ultra HD transmission technology at the Mobile World Congress tradeshow. According to The Verge, the streaming utilizes Wi-Fi and the Miracast peer-to-peer wireless screencast standard in order to stream the 1080p source from the phone, before being upscaled by the TV to Ultra HD.
The demonstration stream included an Unreal engine Android title that suffered in framerate when pushed, but the tech is unfinished. Keeping the phone from overheating is another concern, and LG is touting their streaming capabilities as less power-taxing on the phone at the cost of reducing brightness on the mobile screen.
While it may be some time before a phone can stream a native Ultra HD source, this configuration has possibilities, at least in terms of games. An enterprising developer can design visuals with the ultimate goal of being upscaled, even offering "Ultra HD Upscaling" as an option that calls different art assets, shadow depth, etc.