First of all - create an Adsense unit you want to use as 4-th block. Create channel for it. Then go to the http://google.com/friendconnect Connect your Adsence account with friend connect Click on the Adsence menu in left area of friend connect Choose block youve just created. Generate code and past to your website done. you can see 4 blocks right here and have a nice AdSence Cristmass )))
It's been noted that 1 block - top left - large rectangle - will pay better because only the highest paying ads show. So you should test that first.
while less ads usually earn more, its good to have the option to have the ability of an extra ad block. some might just like the idea.
Test it out and see. The rule of "Less ads pay more" is not written in stone. You might find an area within your website that gets more exposure and more money. It really depends on the layout and niche.
So here the results after testing - for the last 1000 impressions ive got 2-times more click than i have before - 1 click for every 50 impressions
Be sure to let us know what kind of results you get after Google bans you for not following their TOS.