It's a funny thing about warranty that people don't seem to understand.
If a company is offering you another 4 years for $100 they're betting that you won't need to the insurance and you're betting that you will. Now the even funnier thing is that they OBVIOUSLY stack the odds in their favour and probably collect and never pay out on 75% of all the tanks they sell the extra warranties for.
But warranties give people peace of mind. And if you think you're tank isn't gonna outlast the warranty then you should get the warranty.
Me? I think I'd take my chances and not get the warranty.