Only four strain gauges? Seems unlikely... Time to hunt around.

Ahah! Some strange rubbery glue was holding a cable over the second half of each strain gauge pair.

As we found previously in Apple's Force Touch, the strain gauges on this trackpad sense pressure from your fingers without really moving.

With the virtual feedback of the Taptic Engine, the new trackpad has essentially no moving parts, which usually makes for a more durable component.