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.
Sonar sonar1(2,3);
Here is an example with some NeoPixels, just for fun:
Cheers!