According to the post, the Ubuntu user connected three displays using BaseMosaic, backed by Nvidia's v310 drivers and two GeForce GTX 560 Ti cards in SLI. But after configuring the xorg.conf file and adding the supposedly supported fourth display, only three monitors would light up.
"Then I tried installing the v325 drivers after uninstalling all the Ubuntu Nvidia packages," the post reads. "This only worked for 2 monitors, because I couldn't get BaseMosaic working anymore, even not from the Nvidia GUI after enabling advanced options. After a little bit of research I found that v310 only supports up to 3 monitors in BaseMosaic, the same is valid for v325."
Yet in v295, the Ubuntu user was able to get four monitors up and running. So why was BaseMosaic altered to support only three monitors instead? Nvidia explains. "For feature parity between Windows and Linux we set BaseMosaic to 3 screens," said "Sandpit" of Nvidia's Linux team.
What's interesting here is that Nvidia's proprietary Windows driver has features not found in the Linux drivers, but the company will remove specific Linux-only features for "parity." The comment indicates that the three-screen limit has nothing to do with a degradation of quality when using four screens, but a possible Microsoft request/demand.
Oh boy, driver politics. Lock if old.