Hello. i just bought magewell quad hdmi and got some test with it.
All inputs can works at one time - it can be found in sourses as separated devices 00-03.
delay between in and out 5 frames or 100ms with 1080p50.
I use nvidia geforce 1080 for render and output - with all four inputs in work i received about 20 percent of load gpu memory.
I hope, this information will be usable for somebody