Each cluster is running two shards: the overworld and the caves. So you'll have 4 shards running. Since there is no multithreading, a quadcore processor will have no trouble handling the 4 shards simultaneously. The simulation isn't generally all that CPU-intensive, unless there are many complex entities working at the same time - brains are usually the most demanding, so having A LOT of pigs or bees or whatever active at the same time will have an impact. However, it won't affect other shards. Of course, all subject to whatever other stuff you're doing on your PC.