From my own personal experience:
7 port USB hub D-Link DUB-H7 worked just fine for me powering simultaneously not only 4 USB ASICminer Block Erupters but also the raspberry pi.  It has a 3A power supply and 2 1A USB charging output ports.
I'm not sure how many more than 4 block erupters it can handle; I assume by its 3A power supply it can handle a maximum of 6.