This is kind of like asking "How long is a string?"
First off, I've seen a couple of hands on that (seem to) believe that the controller isn't HD and I wouldn't be surprised if the resolution is around standard definition TV; maybe better, but I would doubt it is as high as 720p.
With that said, 802.11n could easily support 4 channels each with the bandwith required to display a SD image; and the rumoured GPU easily supports 2 displays at (above) 1080p each, so I doubt it would struggle with 4 SD displays along side a HD display. The reason why this becomes a much more open ended question is the number of displays it cans support depends on what you're trying to do with all these displays. High Definition "Wii Like" Graphics on the TV with lower resolution "Wii Like" graphics on each of the 4 controller displays shouldn't be too difficult; but I doubt you would see graphics that were better than the HD consoles displayed to the TV and 4 controllers at once.
Ultimately, we can't know without knowing more about the system; after all, Nintendo could have created the system with a dedicated GPU for the TV and another GPU for the controllers or there could just be the one GPU to run everything.