HyperDrive supports up to 2K60Hz or 4K30Hz HDMI video output for USB-C devices in general.
However the 2018 iPad Pro only supports 60Hz video output (720/1080/2K/4K at 60Hz) and does not support 4K30Hz so the highest resolution video output 2018 iPad Pro with HyperDrive is 2K60Hz.
4K60Hz video output requires the full bandwidth of the USB-C port (utilizing 4 out 4 video lanes) so, in order to do 4K60Hz video output, a dedicated USB-C to HDMI video adapter is required. The confusing part now is that most of the dedicated USB-C video adapters in the market have older chipsets which are also not 4K60Hz video output compatible with the newer Parade Technologies DP825 chipset used in the 2018 iPad Pro. USB-C video adapters need to use the new Parade Technologies PS186 or later chipset in order to get 4K60Hz video output from the 2018 iPad Pro. The even more confusing part is that because the 2018 iPad Pro supports DisplayPort 1.4, using PS186 or later chipset, it is possible to make multiple port hubs which support 4K60Hz video output (using 2 video lanes instead of 4) but it would require the Display Stream Compression (DSC) protocol used in the 2018 iPad Pro which Apple has yet to release.
So the long story short is that it is not possible to make a multiple port hub that supports 4K60Hz video output with the 2018 iPad Pro unless Apple releases the DSC protocol. Due to hardware and software limitations, even Apple’s own 3-in-1 Digital AV Adapter (or any multiple port hub for that matter) is not able to do 4K60Hz with the 2018 iPad Pro at the moment.