F4 will not show Dashboard
Unfortunately, the Apple Wireless Keyboard manufactured from 2011 onwards swaps the special Dashboard key for a special Launchpad key. I don't believe this special key function can be changed.

If you have assigned the keyboard shortcut for F4 to Dashboard, you will need to press the function(fn) key while pressing F4 to open Dashboard. Additionally, you need to make sure Dashboard is enabled in System Preferences > Mission Control as either a space or an overlay.