I'm not going to give you a script, since it is really easy to do with events. All you need to do is make a common event and set it to parallel process. Inside the common event, place this:
Conditional Branch: Button <Shift> is being pressed (Actually, you assign Shift to a button by pressing F1 during test-play)
Set Move Route: Player
Change Speed: 5
Else
Move Event: Player
Change Speed: 4
End
You can add wait frames at the end if you feel any lag from it. There is a
Double Tap Dash Button Event System in the
Event Systems DatabaseIf you need more help, don't hesitate to ask
Granted, I am using RMXP, but 2k3 wiwll have something similar, maybe instead of con. branches it would be Button Input Processing?