There are a few caveats to be upgraded to 4K.
From Google's own site:
"Watch movies you’ve already bought in 4K
Some SD and HD movies that you’ve already bought or redeemed might be upgraded to 4K. To be updated to 4K, movies must be:
Bought or redeemed from Google Play or YouTube.
Only SD movies bought or redeemed before October 23, 2018 will be upgraded.
From Fox, Lionsgate, NBC Universal, Sony (US only), or Warner Bros.
Available in 4K, in your location."
https://support.google.com/googlepla.../7184994?hl=en