I think I understand, and from the symptoms you described (one of the psi readings being blank), it does sound like one sensor.
Let me send along a description of the system and the schematic. That way you will have more information to work with these folks.
I am assuming that you are dealing with a GM dealership that has the capability to read the specific error codes that come back from the system, and that has pointed them to a particular sensor. As you might be able to tell from the schematic, these sensors are just pressure sensors that transmit a signal to the receiver. If one of them is blank (like you saw), then that would point to that particular sensor.
So far they have changed 3 out of four, which only leaves one to go. If this was a GM dealership, it would surprise me that they did not get it right the first time, and that they would not have looked for something else wrong the second time (or the third). If this is NOT a GM dealership, and they don't have the right tools to properly diagnose the problem (scanner that will tell them what sensor has bad signal), then perhaps they were just "guessing" and have one shot left.
I believe there is also a calibration / relearn procedure that should be done whenever the tires are changed or rotated so that the system knows which tire to display as the problem. That is why I was asking about the paperwork details. Most shops will list the relearn procedure as a line item on the paperwork as a final step before testing.
I hope this helps and gives you more information to go on. Please let me know what happens or if I can provide more information.
Take Care, Greg A.