Last time I used an HC-SR04 with a Teensy, I just wired the TRIG and ECHO pins together and told the NewPing Arduino library to use the same pin for both functions. I’m not sure where I read ...

The HC-SR04 appeared on the market a few years back as a low-cost alternative of the Ping. [Emil] found that the HC-SR04 would work reliably on hard surfaces as far as 4 meters away from the sensor.