External USB hub for 4 external hard drives
Make sure the hub you buy powers each port to 500mAh. Most name brand hubs do this so you'll have to go to lengths to find a cheap / lesser specification hub that doesn't come with a plug in power supply to not be able to power three devices.

In general, shopping for items that Apple has chosen to sell through online store is a great way to identify which vendors make quality products. The best of the best items get placement in the retail stores. Since that space is very limited only the most useful and cool accessories make the cut.