In a popular book on quantum gravity "Physics meets Philo on the Planck Scale", it is mentioned there are 4 roads to quantum gravity: "1. quantising General Relativity 2. quantising a different classical theory, while still having general relativity emerge as a low- energy (large-distance) limit. 3. having general relativity emerge as a low-energy limit of a quantum theory that is not a quantization of a classical theory 4. having both general relativity and quantum theory emerge from a theory very different from both" Can anyone give an example of the fourth road, this part about having both general relativity and quantum theory emerge from a theory very different from both? Is there really such? How many theories along this line? How viable?