There is a Spears & Munsil UHD disc in the works, but I'm not sure of the release date yet.
I use a program called HCFR, via my PC, it is a calibration software that can generate 4K test patterns. Also because I use a PC, it is still via HDMI and therefore as close to source chain as I can currently get.
But there are plenty of free repositories of 4K test patterns available, which could be loaded onto a USB. AVS forums have a large number of threads dedicated to the sharing of such media.
If you have a 4K player, you could use the USB port on that to display the patterns, which would be the same as if you played a disc through it. But I can't see there being much difference between USB and HDMI on the TV directly, as both are digital and should show the 1s and 0s in the same way. Unless the TV or player is tampering with the material somewhere in the chain.