Sounds like a plan:

• Have an weight you're not happy with
• Bet >$4000 with a stranger
• Go to the doctor, get your stomach stapled, undergo lipo, hire a personal trainer or get prescribed (good) diet pills => lose the weight you want to lose
• Let the stranger pay for your treatment, earn a little extra

I do applaud your quest to get healthy, but like it's been said before: it's something personal. The person taking this bet would have no control over your actions, so he'd be in a disadvantage straight from the start.
There are other forms of motivations...

Good luck tough!