Will Tyranitar stil be a go to Rock Mon in Gen 4?
Rampardos and Rhyperior outclass it on the DPS and TDO ends respectively. Suppose Niantic gives both optimal sets, is it worth keeping Rock Ttar?
Answers
That's considering it even gets double rock moves. In the normal games it learns zen headbutt and they could give it a normal type move like headbutt. My guess would be they don't make it double rock. They could give it a charge move like head smash, a rock move that is stronger than stone edge, and a quick move like headbutt, zen headbutt, or iron tail.
The better question is whether you need Rock types in Gen 4, and the answer to that is mostly no. It'll be good for Articuno Day (and Zapdos/Moltres Days) and is the best counter for non-FB Regice in Partly Cloudy weather, but those are the only uses for it in the foreseeable future.
The article on Rock types on this site goes into detail about the Ttar/Rampardos/Rhyperior breakdown. We'll find out in several months where Ttar stands, as it depends on their moveset. The data on that page assumes all of them will get SD, which seems unlikely to me - without a Rock quick move, the conversation is moot and Ttar is king. Assuming optimal movesets, it looks like Rhyperior is slightly better all around and Rampardos is much better DPS-wise.