What is the Mod4+D shortcut key?

In System/Preferences/KeyboardShortcuts there is a shortcut key called Mod4+D that can be used to minimize windows. How is Mod4+D entered on a keyboard and what is the meaning of the Mod4?

You can see what keys are bound to which modifiers using the xmodmap tool. For example:

$ xmodmap
xmodmap:  up to 4 keys per modifier, (keycodes in parentheses):

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x42)
control     Control_L (0x25),  Control_R (0x69)
mod1        Alt_L (0x40),  Alt_R (0x6c),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3      
mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce),  Hyper_L (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Mode_switch (0xcb)

The Super_L and Super_R keysyms correspond to the Win keys on your keyboard.