That kind of offer isn't itself surprising, but it being available to existing Netflix subscribers is unusual.
> You may apply the value of the offer to a different Netflix streaming plan.
From the terms of the offer, it does look like you can apply the value of the offer to a 4k plan at a reduced duration.
* Offer value may be applied to a different Netflix streaming plan under the same account; exchanges in this manner may alter the duration of the offer. Netflix price plans subject to change and the duration of the offer value may be prorated accordingly.