The usual way to reference a sheet is [here][1] but how do you reference a sheet when the sheet's name is a value in a cell?

For example, 

    A1 = "Sheet number two"
    B1 = A1&"!B4"

Here I want B1 to display the value in `'Sheet number two'!B4` but it's not working for me.  Any ideas?

  [1]: https://support.google.com/docs/bin/answer.py?hl=en&topic=1361471&answer=75943