SC4 base files are re-distributable...
"All add-ons based on spacecraft4.dll may be distributed freely with this base pack
included but you should preferably ask to download last spacecraft4 version"
I feel tempted to create a basic satellite add-on and upload it named "Vinka's spacecraft demo"... that way people can find spacecraft.dll on OH.
Did just that!
For some reason there's no corresponding discussion thread for that add-on, but really, it's just a simple cubesat setup with spacecraft4 along with the entire SC4 package.
Should be useful specially for new members.