A router is better cause you can share stuff via wifi as well (nice for phones & tablets)
Most have 4 ports, so if you want a PC + 4 media players you're 1 port short - either:
A) Connect 3 + PC via cable and the 4th player via wifi.
B) or shell out for a switch.
I'd go with option A.
You can only stream 2 movies at a time from your PC, i'm guessing 3 is gonna be too much.
I reckon if you connect some of your hard drives to the other media players though, that you will be able to get out a lot more, as the HDD bandwidth is "split" better
(The ones with the drives attached must obviously be powered on to access everything from every room though)
I'd say connect a drive or two to ONE OTHER media device, and always keep that one one - maybe the living room one
(Use your "best" player there, that could be able to connect to 2 hard drives)
*Im not sure what players you have and if they can take internal drives or if you even have external drives, it's just an idea to share the load. (my theory, at least)