I downloaded Unity 5. But now I can’t access my old project from Unity 4! It won’t open at all! Should I just uninstall Unity altogether then re-install Unity 4?
You can try to import old projects but sadly there might be a lot of errors because there’re huge changes in Unity 5. The only way is to have 2 versions of Unity. But to be honest, try to use Unity 5. It’s much better.
Try using the "Open With’ function and select untiy 4