Could I count traffic with the help of mirroring port on a Catalyst 4503?

You could span the port to a spare interface on a linux based host and use an opensource netflow probe. Tie that in with something like NFSEN and you could acheive what you want. Although you would obviously need a server to acheive this.

http://nfsen.sourceforge.net/

http://www.mindrot.org/projects/softflowd/

Ben, the softflowd generates netflow traffic to a server that collects it, does it?

Do you know whether I can install the 1720 instead the linux server with softflowd?

Do you have a layer 3 supervisor in the 4503? If you do, post what it is and then we can tell you if you can use it for netflow.

Yes, I have the layer e supervisor II+TS. But I know, that it doesn't support NetFlow.

I thoght about spanning the port to a spare interface. I have two opportunities: to use a linux host or an old router 1720 (it's not used in our network). Is it possible to use the 1720 as a netflow agent in this situation?

I have never done it that way, but if you look at Bens post he has.