Note that this doesn't mean you get 8 years of warranty on an expensive oven. Just that if it fails in 4 years you still can claim 50% of the purchase price.
If, however, for whatever reason you don't want that, you can't demand all your money back, but only 50%. That's only if you agree to the money though, the seller can't unilaterally choose to give you 50% back instead of repairing it.