Is there a way to force a global flush of DNS caches ?

Hi Folks,

I have an unusual problem.  I have a web-site that is hosted by a 3rd-party hosting provider. I recently switched hosting providers, and had to update the DNS records to point to the new hosting provider. The TTL on my DNS records (before and after the update) was set to 4 hours.

It has now been 3 days since the change, but a friend in another city pointed out to me that my site isn't loading for him. Upon troubleshooting, I found that my friend's machine was still resolving the site to the old IP. And I've been told by another person in a completely different location that they were seeing the same thing, so it seems that some DNS resolver in some part of the world still hasn't updated their DNS cache. 

So I'm wondering, is there a way to force a purge / flush of outdated DNS records globally? If so, how would I go about doing so? 

Thanks!