Can you just delete Rome Total war and the Fourth Age total war mod

Yes... and no. The best way to remove an application is to use the add/remove programs tool, typically found in the control panel. You can also just delete the folder, typically found in the program files in your default drive. This second course of action will leave lots of registry entries and maybe some folders in your my documents or my games folders, but will remove the bulk of the program.

In the end I highly recommend running the uninstaller (which is what using add/remove programs does)