Nearly all of the mid-to-high end Kepler based NVIDIA cards (the 600 series) support 4 monitors on one card.
Check out the specifications for any card you might be considering to see if that one does support 4 displays. See where is says :
Multi Monitor 4 displays
That said, I only use the NVIDIA supplied driver and it does support a unified desktop (Twinview, even with more than 2 monitors, is what they call the one-desktop mode) and rotation of any/all monitors. Use the nvidia-settings tool to set the rotation and monitor positioning. Run as root to save the changes permanently.
Pre-Kepler GPUs, the 500 series and earlier, do NOT support more than 2 monitors per card. You'd need a multi-gpu (non sli) setup to get that.
I should also add that depending on which desktop environment and version you're running, you may have issues with 3-d acceleration on very wide desktops. This I don't know for sure, but I recall reading about issues with earlier versions of Gnome3 and desktops wider than 5000px. This issue, if it ever really existed, doesn't seem to any longer, or, at least not at anything close to 5000px in general. BUT lower-end GPUs cannot do 3D acceleration across very wide desktops. All GPUs have limitations on how large an area pixel wise they can perform 3D acceleration but documentation on these limitations is not easy to come by. My current desktop is 5440px wide is is fine on a GTX 670 (and would be on anything 650 or above. that's a guess, though).