It looks like two threads were merged so the flow of comments is a bit confused, but I gather that the HC-SR04 modules that look like this are 5 V only (these are what I have on hand):
The HC-SR04 modules that look like this apparently will work at lower voltages per "wvmarle" post. Note the designation on this board is HC-SR04P:
As always, when ordering on eBay one may not get exactly what was pictured in the ad. Regardless I've ordered a couple of the latter design with the plan of trying them out and reporting back when I get them.