PC Magazine has an article up that describes the Pro as having an Oled screen, and a Realtek chip embedded in the dock that provides the 4K magic. It looks like it still offers the same 720p display, and the same Mariko chip in the Lite and newest revision of the Switch. Looks like they source VGC and twitter users as discovering this in the Nintendo Switch Firmware.
Link here.PC Mag said:We expect the Swicth Pro to offer 4K visuals, and the firmware also suggests this is true, but not on the Switch handheld itself. SciresM found reference to a Realtek chip in the firmware, which is a "4K UHD multimedia SoC." It's thought that Nintendo intends to add the 4K SoC to the dock of the Switch Pro, allowinbg for 4K output through a compatible TV