I agree with Mike ML that an ultrasonic approach may be the least complex method.
You could have the first unit transmit an ultrasonic pulse, and the second unit transmit a pulse (at a different frequency to differentiate it from a reflection off another object) when it detects the first pulse. The round-trip time it takes between the transmit pulse and the first unit receiving the return pulse from the second unit can be converted to distance based upon the speed of sound of about 1ms/ft. Thus if the time is less than 8ms, then the operator is less than 4 feet from the equipment.
In theory you could do the same thing with light or radio frequencies, but the time interval is much more difficult to measure, since the speed of light is about 1ns/ft.