Q: VW Polo running on three cylinders?

This is possibly a sign of an intake air leak creating a lean fuel condition. When this happens, this causes an imbalance in the air/fuel mixture inside the cylinder causing the misfire, which can also cause the engine idle to fluctuate as you have described. As the electric pulse continues to be delivered to all spark plugs, the remaining cylinders fire normally as the proper air/fuel mixture provides the perfect explosion inside the cylinder. When the 4th cylinder receives the same spark and fuel, but additional air, this leans out the mixture due to excess oxygen being added to the fuel being ignited. I would recommend having an expert from Your Mechanic come to your location to take a look at the vehicle to properly diagnose the idling issue and repair.