The data sheet describes all the interfaces available. One is uplink to the controller, 3 are downlink for use of wired connectivity through the uplink. The black port on the bottom is basically a shunted, wired connection from the back to the bottom in cases where the uplink port in the back cannot be used (desk stand, uplink wire is not coming from the box behind it, etc).
So there's only 3 ports to use for wired devices connected to the 303H. They can obviously put a switch there for more than 3, but I don't think they want switches tied to the APs.