make use of the 5v pin on the ESP8266
Well, an esp8266 chip does not have a 5V pin. Esp-12 modules don't have one either. So what are you really using? A NodeMCU? A Wemos Mini?
If it's a Wemos Mini, that has an ideal voltage regulator, from a Li-ion point of view. Unlike a simple diode, it will drop just enough to reach 3.3V, and when the battery voltage drops below that, it will only drop 0.2V. However, it cannot create 5V, and nor can any other regulator.
Ultrasonic sensors are one of a few remaining popular Arduino components for which there is no cheap and commonly available 3.3V equivalents. We as a community should be looking to do something about that. Finding, recommending, exercising our group buying power, that sort of thing.