Just finished writing a small C++ class to work with multiple HC-RS04 sonar transducers with a non-blocking way with the NewPing Library.
In this exact code I connected two sensors, but you can add as many as you like.
To add new sonars, just create a new instance of it adding the echo and trigger pins.
Here is an example with some NeoPixels, just for fun:
During the First Meeting of Electronic Arts in Florianópolis, we built a Breathalyzer using the Alcohol Gas Sensor MQ-3 and an Arduino Board to use in the last day of the meeting, in which we gave a party. You can see a quick video two posts below. Last days I received many emails asking for the code or how to make one, so I decided to build the sensor again, take pictures/videos and make a tutorial showing how you can make one, so here it is.