Could be that you have 4 installed but the icon in the dock is pointing at version 3
If you find the Version 4, remove the existing icon from the dock, by dragging it onto the desktop, and then drag the V4 down into the dock.
Let us know if that fixed it
When I attempt to install Safari 4 on my Mac OS 10.4, I get a message saying it cannot be installed, and a newer version is previously installed. But when I start Safari I'm getting version 3. How do I determine what 'newer' version has been installed and execute it when I click on the Safari icon?