While I couldn’t find the Spanish installers on the Wayback Machine, I was lucky enough to find them in @enhanox’s MSN client archive (MSN Messenger Clients : Microsoft : Free Download, Borrow, and Streaming : Internet Archive). I have packaged both the Spanish and Portuguese versions of the MSN 4.6 installers in a ZIP file for ease.
MSN_4.6_Spanish.zip (3.6 MB)
Enjoy. 