I'd go with option 1 personally and look to leverage the content another way...
I love the "Pay with a tweet" idea, there are services which require you to Pay for a download with a tweet. http://www.paywithatweet.com/
This is a great way to get some credit for your work, spread the word socially and attract some links/authority from relevant sites and its all perfectly natural!
Option 2 sounds a little shoddy and difficult to manage.
Option 3 could prove to be dangerous with hundred/thousands of ;links with same anchor text
Option 4 Woah there buddy, this sounds a litle 'grey'.
If you want to be 'Completely clean' go with option 1, ask people to share socially or link to you in return for the content.
Hope thats of some help.
Aran