How to fire flash multiple times during an exposure?

I have a Nikon D5000, and I was wondering if there's any way to fire the flash say, 3 times during a 4 second exposure. I only have built in flash. So no, strobes or attachments do not apply.

The easies way will be to activate the Red eye elimination option in your camera.

That way you will always have multiple flashes if you are shooting with the built-in flash up.

No there isn't. You can fire it at the beginning of the shot (front or 1st curtain sync) or at the end of the shot (rear or 2nd curtain sync) but not multiple times.

You could pick up a cheap manual flash off ebay and fire it during the 4 sec exposure using the 'test' button. Any old flash would do for that. Its known as an open flash technique.

Edit: the recharge time will depend on the power the flash is fired at. My Yongnuo 460-II recharges in about 0.5 sec when fired at half power with fully charged 2500mAh NiMH batteries.

Well true, any old flash will do providing it recharges in under 1 second which most don't. But these fast flash units are available. I agree, this is the only way.