There's a lot more to paper than you might think—not just the colour and size of the sheet, but also thickness, texture, and pattern. A4 card comes in all conceivable variations, the only constant is the surface area.
Among the other differences, keep in mind the GSM, or grams per square meter, given in the listings. This indicates the thickness of the A4 card. Regardless of what you need, there's an A4 card just right for your project.