just check this page. http://pocketnow.com/android/lg-optimus-3d-review it has 5 ad units. how did they do that?? how ever it has stupid ad placement :B
Usually the 4th, 5h and so on ads are actually coming from an ad network they have. That is, they put 3 adsense units put 3 CPM ones, but the CPM networks sometimes also display AdSense to fill inventory. I am not sure if this is allowed by AdSense's ToS, but that is what happens.
I think that is ok with google policy, google only advices for not to use more than three as most valuable ads will be there in three blocks and more ad blocks can make your site unfriendly for readers. however, it does not make much difference on income. More ad placement do not mean more revenue.
If it's all adsense then they're either breaking the TOS, or have a 'special agreement' with big G as they have major traffic, that is my understanding anyway!