USB 3 solution: Blackmagic Multiview 4 ($495)+ Blackmagic Smart Panel ($85) + Magewell USB Capture SDI (or HDMI) 4K Plus ($459) - Just make sure you turn all the overlays off and you can combine (4) 1920x1080 HD into (1) 3840x2160 UHD signal
Thunderbolt 3 solution: OWC Mercury Helios 3 Thunderbolt™ 3 PCIe Expansion Chassis ($199) + Blackmagic Decklin Duo 2 ($495) - I have used the OWC Mercury Helios 2
Thunderbolt™ 2 Dual Slot Expansion Chassis For PCIe Cards for years and it works great!
If you need to ingest HDMI, you can add (4) Blackmagic Micro Converters HDMI to SDI ($55 with power supply or $39 without)