HDTV as Split Screen Monitor
"Can I connect a computer to a 46" HDTV and use split screen software to make it like four screens / monitors in one? I have a PC with XP, and a Mac with OS X Leopard 10.5. However if the solution requires a different OS, I can get a new system."
There are a number of ways to achieve displaying multiple screens on the same physical screen. If you were to run 3 additional operating systems in virtual machines on Windows, you would effectively get 4 unique screen displays. On a Mac running something like VMWare or Parallels you could increase the number of screens. While the technology to do this is certainly possible, you're going to run into some limitations beyond the operating system that may be frustrating for the result you attempt to achieve.
Any PC with support for virtualization will allow you to display 4 screens side-by-side. On Windows either Virtual PC or VMWare are great solutions for running 4 instances of an operating system (or 4 different operating systems), at which point you would just need to move the 4 windows to their own quadrant on the screen. VMWare is likely going to be the best solution for this type of project on a Mac. If you need one of your 4 screens to be a Mac, it might be best to use the Mac as the source for the other 3 screens, since there's no reliable way to make sure you can get Mac OS to run in virtualization on a PC.
One thing to keep in mind as you split up your screen is that you're not gaining 4 smaller spaces of higher resolution. So when splitting an HDTV into 4 equal screen spaces, at best you are taking a 1920x1080 single screen and effectively making it four 960x540, which may be too small for some types of applications. If your HDTV is 1280x720, then you become limited to 4 640x360 spaces which will definitely be too small for most PC and Mac applications.
Another possible way to approach this problem is to look for HDTVs that support display of multiple inputs simultaneously. Some HDTVs allow you to view 2 HDMI connections at the same time for instance (I'm not aware of any that support four screens, but they may exist).
In the longer term, Microsoft Research in India is working on an interesting split screen technology which allows you to connect multiple keyboard and mouse interfaces in order for multiple users to use the same screen independently.