My player has a 4e Pixie Monk and his Daily power says he can shift his speed (which on his sheet says Speed 4 Flight 8) and I'm not sure if the card is specific to Speed or can he use his Flight speed to shift for say Whirling Mantis Step?
You use the speed of whatever movement mode(s) you are using
For most intents and purposes, all of a creature's movement modes are interchangeable other than what sorts of terrain they can be used in. All of the various available speeds simply define how far you can move while using that movement mode. i.e. if a creature is flying, it uses its fly speed for any movement it takes.
The Rules Compendium has information supporting this conclusion spread out in various places through the "Movement, Distance, And Terrain" section of Chapter 6.
move: Any instance of movement, whether it is done willingly or unwillingly. Whenever a creature, an object, or an effect leaves a square to enter another, it is moving. Shifting, teleporting, and being pushed are all examples of moves.
speed: The distance (in squares) that a creature can move using the walk action.
Rules Compendium, page 200
Special Movement Modes
If a creature can use a special movement mode, it can take the walk or the run action using its speed (or simply stand still) with that movement mode instead of its walking speed. It might also be able to charge, shift, or use other move actions while using that movement mode. For instance, a creature that has a fly speed can take the walk action to fly instead of walk, and it can shift while flying.
Rules Compendium, pages 202-203
Fly Speed: To fly, a creature takes the walk, run, or charge action but uses its fly speed in place of its walking speed. A creature that has a fly speed can also shift and take other move actions, as appropriate, while flying.
Rules Compendium, page 210
As a side note, it's possible to combine multiple movement types into a single move. Rules Compendium page 204: Combining Movement Modes covers how to calculate your speed when doing such.
Rules Compendium on Speed:
A creature's speed represents the number of 5-foot squares it can move when taking a move action to walk. Some creatures have speeds for movement modes other than walking: Burrowing, climbing, flying and swimming.
This would lead me to understand that an effect that refers to a creature's Speed refer to the actual Speed statistic, and not it's speed when using special movement modes.
Basically, 'Speed' and 'Flight Speed' are different terms, and thus mean different things.