Electronic – Does it exist: 360 degree ultrasonic receiver/beacon combo

A typical piezoelectric ultrasonic transducer won't be as efficient when driven at frequencies other than it's rated one. They act a bit like a resonant LC tank, so sensitivity drops off quite sharply.
A typical figure seems to be ~2kHz -6dB bandwidth (e.g. with 40kHz transducer if will be half as sensitive or give half the output at 39kHz and 41kHz)

40kHz seems to be the most common (and cheap) but you can get different frequencies like 25kHz, 60kHz, 180Khz, etc.

Rather than doing this though, why not just use 4 of the same and pulse each one separately, or use one and rotate through desired angle with servo.
Unless your robot is travelling very quickly it shouldn't matter if you stagger the pulses slightly. You can even make a basic SONAR this way, here's an interesting PIC based example.

The sensitivity of ultrasonic transducers does vary over frequency. The following example is from a Kobitone 255-400PT16-ROX datasheet. It would also be fair to assume it would fall further outside the normal operating frequencies that are shown below.

However in my opinion using it do determine distance isn't likely to be a good solution. When operating outside normal ranges you may find differences in performance between parts but for experimental / hobbyist purposes I see no harm in trying it as a simple solution.

Another way you could go about it is to change the amplitude of the signal at the ultrasonic emitter end and encode it with a digital signal that indicates the transmission level used. That does get quite a bit more complex though. If you wanted to take a look at going that way perhaps the VirtualWire project would be worth a look at for encoding and decoding the digital signal.