Is buying a Premium subscription the only way to unlock 4K Downloads?

@ezromeo
Yes.

Do I have "Re-download" the cards I want to see in 4K? If so, how is that done; and am I spending credits to do that?

Must be premium (with subscription) to have access to 4K resolution. Without that, you can still see the 4K cards but will not be able to download higher than 3K.
When one becomes premium it is not necessary to download any cards in 4k. This is a choice.
Once the subscription has ended, cards cannot be downloaded in 4K.
After the end of the subscription, cards downloaded in 4k stay that way.
Downloading a card in a different resolution (even 4K) does not cost any money beyond the initial expense of making the card purchase.

@ezromeo
The re-download is free of charge. When you will be Premium, open a card page and click on the resolution box. There you can choose the 4k resolution and then re-download the card.
If you want to re-download more than one card in 4k, go to My Collection tab, select all the cards you want and at the bottom of the window you can change the resolution for all the selected cards (this method is for the stable 1.2.552 version you use).

Do I have "Re-download" the cards I want to see in 4K? If so, how is that done; and am I spending credits to do that?

Also want to Add:
When you become Premium
you can re-Download all cards you already own, that are Crystal Clear Plus in 4K with no charge.
and once you get the 4K version, it will remain available in 4K even after your Subscription ends normally.

However,
If you Cancel your 4K subscription and get a refund, then all access to your 4K resolution is removed, Including access to any 4K clips you already have downloaded. ( you still own the card, but not 4K resolution of it, you will have to download in 3K or Less )
The Main thing here is If you Got a Refund.

If your Subscription ends normally, then you KEEP all the 4K resolutions you have.
but you can't download any new cards in 4k, 3K would be the Max after the Subscription ends.