@CrimLiarthe founder's edition is not a subscription.
Stadia Pro is a subscription. As mentioned before you get 3 months Stadia Pro for free with the Founder's edition and some bonuses on top (a Founders name (unique username that will not have numbers appended to it), limited edition Night Blue controller, 3 Months Stadia Pro and a Buddypass).
Regarding the 4K, this is bound to the Stadia Pro subscription, so as soon as you don't want to pay for it anymore, you'll lose access to 4K. Just good to be aware of, but at the end of your 3 months Stadia Pro for free as a Founder you can decide if you still want Stadia Pro further or if you cancel it and use Stadia as normal.