Henni

Sorry, I'm a little confused. You have over 4GB of files on a 4GB card? Well I suspect not, as that would be impossible.

As far as being able to write to the card with Windows 7, what exactly is the error message?

I hope you checked the obvious, and it isn't write protected.

Another thought, Windows 7 can use the card to speed up your your laptop (Ready Boost), and if you enabled that, you can't write to the card.

Make sure it's not checked in Properties.