teckel:
Could I have one of the defective SR04 sensors?
Hm, sorry, I suspect it'd be an expensive test sending it from Italy to the States.. 
Especially considering italian postal services efficiency..
teckel:
Also, if you have multiple sensors you're using in the same project, have you considered using the ping_timer() method with NewPing?
No, I'll give it a try, but I think it won't make any difference because the problem seems to be on hardware side, and I don't have particular delays, the (in)famous "one second timeout", if I set a few milliseconds as a timeout (e.g. 50.000).