Apparently the display isn't wireless, it just talks to a dedicated GPU in the base when the screen/tablet is docked. Should reduce latency, but unfortunately the GPU would be nice when using Photoshop et al. in tablet mode. The screen can also be flipped and attached backwards, for a faster--if unwieldy--tablet experience.
I wasn't too impressed with the pen demo on the SP4, seemed rather laggy, like what you'd expect from an iPad. Still waiting to see if the iPad Pro 'Pencil' is half as smooth as Apple's videos made it look.