On the question of 40kHz interference amongst multiple HC-SR04 units, John's idea of a detector link that would stagger the transmitted pulse outputs among multiple units is intriguing.
Another alternative would be in the physical realm. There are a multitude of inexpensive porous acoustic absorbers which should be effective in this frequency range. For example, mineral wool, natural fabrics, etc. At this frequency, it should require little material thickness to effectively adsorb the propagated signal and prevent nearby detectors from interfering with each other.
Stan, would you be interested in seeing if this theory is applicable by placing something like a couple of layers of denim between the detector and another reflective object to see if the signal is adsorbed?
