The actual design of the s4 card is unknown. It is not known how exactly the flag will be cropped and if parts will be obscured by other elements. However, the height and width should be very similar to the one assumed in this preview.
How to Use
You will need a userscript manager extension like TamperMonkey. After installing it, you can click the userscript url and it should have an install button.
Visit the upload flag page and enter a nation name, or upload a flag, and click preview flag instead of submitting it. The caveat to the latter is that this is pre-compression.
You can also select a rarity to preview the card in. If you want to use the third-party cards rarity calculator, you can input your information here to see what you might be come season 4.
For example, this is what I would see
Technical Stuff
The site loads the deck css stylesheet (6.5kb, uncached), in order to render the cards. When submit/preview flag is hit, the script sends a request to the nation's HTML page (in order to get the flag, and also gets whatever is easy to query select). Links to render images for badges are also pulled. The css sheet and images should be cached after one load if not already cached.