Not sure why that increase. $4.99 is the ad-supported, $9.99 is the ad-free.
Roku doesn't set prices if you are paying through Roku. They just pass on what the services charge.
If you go DIRECTLY to the Peacock WEB SITE, they are offering a year for $19.99 if you pay them DIRECTLY, which is ALWAYS much safer and usable on ANY device.
https://www.peacocktv.com/summer-of-peacock