5 channel video installation
A Macbook pro would be about the most expensive device you could possibly use as a playback machine, but I'm guessing you didn't buy them for the installation…

There are several devices that are designed specifically for the purpose of video playback, for example these units by Brightsign. They're used in the museum where I work, and are popular for retail display. They can synch across networks.

An emerging competitor is the Intel Nuc, which his probably a bit cheaper than the brightsign. They're a bit more general-purpose than the brightsign, but they're becoming popular for exhibition displays and signage. The latest major installation that I did at work was a blended three-screen, seven audio channel video installation plays smoothly on a NUC with synched lighting effects. I'm not sure what the software is that runs it all, I just made the content.

Going even cheaper there's the Raspberry Pi, basically it's a small very cheap computer that has HD video playback capability. Whether there is software to do what you want it to I don't know. I know they are also used at my museum for interactive video displays, but I don't know if you can connect several to synch playback. VLC might be able to do it.

You'll probably find more options if you look for digital signage players. Like this one, about which I know nothing.