That's one old computer my friend. I think I have one of mine still around from years an years back. Anyways, you won't be able to support 4 monitors with a 7800, no graphic card that supports that many monitors is even compatible with it anymore. At most you could perhaps buy a second hand ATI Radeon 2400 HD 256 MB PCIe which supports two monitors. However you might have to upgrade your power supply as well. If your willing to do that let me know.
However there isn't much else you can do, hope that cleared it up. Select a solution if you don't have further questions. Take care.