It's been half an hour, just waiting a bit will yield better results.
It's a 4 layer PCB because if you made it with two layers the entire track before the loop would act as an antenna and otherwise mess with the response of the probe, even holding it would effect readings significantly. With four layers, the outer layers (you only really need 3, but 3 layer boards are uncommon) act as a shield, and when stitched together with vias, effectively create a coaxial cable within the board (the signal track is surrounded by grounds).
The track on layer two is stitched to the connector center pin pad on the top layer with vias on the left of the image.