YouTube TV started as an affordable alternative to cable TV services from Comcast or Spectrum. Over time though, the service only got more expensive. The cable TV alternative launched in 2017 for $35/month but currently costs $65/month. Worse, that's before the 4K add-on, which costs another $20 per month on top of the base plan. To make things a bit more affordable, YouTube dropped the price of the 4K tier to $10 monthly for the first year. Seemingly, Google's 4K add-on did not find many takers, which is why the company now offers it at heavily discounted prices to selected YouTube TV subscribers.
There are multiple reports on Reddit from YouTube TV subscribers about the 4K Plus plan being available to them for as low as $5/month for two years (via 9to5Google). Some users are seeing a slightly lower discounted rate of $7 or $10 per month for the same period. Once the offer ends, YouTube TV will go back to billing its 4K add-on subscribers at a monthly rate of $20.
At such heavily reduced prices, YouTube TV's 4K Plus tier makes a lot of sense. Besides 4K streaming of live and on-demand content, the add-on plan allows you to download DVR recordings to mobile devices for offline playback and unlimited streams within your home. On the base plan, YouTube TV subscribers are capped at three streams.
It is unclear how Google is determining the discounted rate for YouTube TV subscribers. Many new subscribers report seeing the $5 offer for 24 months, while long-time subscribers are getting a discounted rate of $10.
If your YouTube TV account is not showing any discount for the 4K add-on, there's not much you can do except wait and hope that it appears eventually. Alternatively, consider using the YouTube TV referral program to earn $20 monthly credits toward the base plan.