It depends on which email app. For the Samsung email app, tap the right-facing arrow next to the trash can, above the message (this is how it works on the S3, and I would guess the S4 is the same).
For Gmail, tap the 3 dots at the right edge of the line that shows the sender, then choose forward. Those 3 dots are Android standard shortcut for a menu, although lots of apps haven't adopted it yet.