The 4 finger down gesture does activate all window Expose by default, and four up activate Expose show desktop mode. This only applies to newer Mac laptops without a separate mouse button (past 2008 or so), or a desktop with the Magic Trackpad.

And just to clarify the F3, it's default on Mac keyboards when the function keys are set to adjust volume, brightness and media playback. On an attached other USB keyboard, or an older Apple keyboard without the Expose icon on the F3, it defaults to what is shown in System Preferences, F9, F10, and F11 for the different functions. (All windows, app windows, desktop).

Quick tip on the keyboards with the dedicated Expose button, it has some extra functions with key combinations:
F3 : All window Expose
Control-F3 : Application window Expose
Command-F3 : Show desktop Expose
Option-F3 : Open Expose system preference (show me the options)

Option and the other function keys will also open the appropriate system preferences.