$$$$$$. As an earlier post suggested, Dish wants to put a limit on how much money they are going to spend per subscriber. The limit is for LEASED boxes/tuners, meaning the tuners that DIsh pays for either whole or in part for you to have. It seems a reasoanable limitation becasue they really don't know if you are going to leave them immediatly after the committment. A few long-time, high-spending customers MAY get an exception to the rule (several MPEG2 HD customers were granted an exception to the rule because they had already paid full-retail for and owned the outdated boxes, and Dish wanted to make good on the customer's investment), but this may be a sub they feel isn't going to leave them soon. But as a rule the 4 leased tuner limitiation is pretty firm.
Of course, you are welcome to purchase as many boxes/tuners as you wish to add to your 4 tuner system, (limit of 6 boxes per household account), but you will have to pay for any additional installation or new LNBF's or switches to make it work. Alos, 4 612's count as only 4 tuners--eventhough they are dual tuner--because they are desighned to serve 1 HDTV and are the solution for subs who want true HD on more than 2 HDTV's.