Multiple Ultrasonic Rangefinder Question
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.