The 4 BNC Connectors of the AJA are for the different SDI Formats, like 6G SDI where you might need 4 x BNC to get the full Signal.

So it's just 1 x SDI in in total (from SD-SDI up to 6G SDI)

I did the testing just with a Macbook Pro, But I think that a faster machine would not reduce the latency of less than 2 frames..

Best

Rolf