Rehydrating is always a good idea with dry yeast, and it won't hurt anything as long as you sanitize whatever container you use.
However, did you check gravity and make sure the yeast you pitched didn't finish the job? I've had many brews cease visible activity after four days and turn out great. Even if you underpitched, the yeast may have multiplied to get the job done.
Read my brewing blog here! Last of the Zyme Lords
BOUSIRIS BOUERY - Home of the Zyme Lord
The cans of Leinenkugel's had masking tape on them with JOHN'S printed in ink. Amy looked around me and said, "John is protective about his beer, isn't he?"
"I put that label on there. When company comes I want them to know the Leinie's is his, not mine." - David Wong
Anna Livia Irish Stout
Melancholy Bill's Infinitely Sad Pumpkin Ale
Lucian's Landing Ginger Metheglin