The regularity of this service, every 4 years, is based on age rather than condition. For those that have gone over the typical time period for flushing the coolant, there are several problems that arise.
One of the more damaging side effects of irregular coolant flushes is high silicate drop-out. This scary sounding problem describes the sludge that forms in a cooling system. This sludge hardens and clogs the radiator, the heater core and the passages throughout the system, drastically decreasing it's ability to function.
Also, coolant breaks down over time. The pH level can become more acidic and begin to eat away at the different metals in the engine, such as the metals in the gaskets, various seals and the aluminum in the cylinder head and to a lesser extent, the cast iron block. Some BMW motors have aluminum blocks which don't respond well to high acidity levels. Typical problems at this point are radiator, heater core or head gasket leaks.
Over time, the performance of coolant also begins to degrade. The range of temperatures it will protect against decreases. If the protection levels falls low enough, in winter conditions the coolant can freeze which can crack your block. Or alternately, a car can overheat in the hot weather.
If you're not sure or can't remember when your coolant was last flushed, it would be a good idea to contact one of our service advisors. The most common times to have this service done are early Fall or Spring, right before winter temperatures and the heat of summer.