Is unity Pro is pro ?

That's a pretty indepth question; the short answer is yes; Unity Pro can be used to create something akin to GTAIV. In fact, it can do just about anything; it's how you use Unity that determines the quality of the games that it produces.

It should be able to, the main thing is can you create the assets(cars, characters, etc.) that show up on screen. In the end, Unity handles the coding

His question's more multi-layered than that, I think. Elliot Bonneville is right that it can do whatever you want it to as long as you're using it skillfully enough, but I think part of his question also concerns Unity as a professional tool insofar as whether professionals actually use it. The short answer to that is, again, yes they do. But only small-scale businesses and private developers looking to earn what they can on mobile platforms. Large scale game developers such as RockStar who developed GTA don't use Unity, and would very likely never use a system that JIT-compiles its code.

Large scale production of industrial standard video games is done in native C++ and sometimes even assembler, because they face out-of-this-world performance/stability requirements you and I have a hard time wrapping our heads around, so they need access to the very guts and glory of the system. In addition to this, they develop on specialized equipment you usually don't have access to unless you're licensed to the platform you're developing on. To make a PS3 title that actually ends up on the shelves in stores, for instance, you need Sony's Reference Tool to test and debug on, which is a piece of hardware that looks like a VCR, with a bundled SDK. It costs about $10.000 in Europe and a bit cheaper elsewhere.

So to summarize: Can you make GTA-grade titles in Unity? Yes, if you're a total wizard. Do the big companies do so? No, because they are archwizards.

Yes Unity can create large scale games that looks beautiful, although you probably don't want to get in on such a project alone. The biggest work in a sandbox game is content creation and architecture, to then make rules for the world is pulled off relatively easy with Unity. The most important thing in large scale worlds are optimizations such as occlusion culling (which comes with Unity), level of detail (LOD) and ranged areas where you disable anything outside the "player zone".

When it comes to coding the AI and the street rules are probably the most difficult part. Cars that take notice of other cars and obstacles, stop and perhaps turn in cross sections. But it's not impossible - it's just the very detail of everything to blend with the rest that might become tricky, the different states of different situations.

Animations is another thing that is important, here you'd probably want a mo-cap studio which is rather expensive. To import and use them in Unity is very easy along with switching animations for different situations.

For example I've coded earlier a vehicle switching logic in Unity where the player sits in one vehicle -> gets out -> chooses a different vehicle -> gets in -> drives. This was just a matter of a few hours (4 to be exact).

So with time, patience and skilled people to work with, you could certainly create an even better game than GTA IV - it really depends on the craftsman cause the tools are already there.

In short - Yes.

In long - Yes. If you get a source lic you could probably do some very impressive stuff and in honesty I've found Unity capable of pretty much everything I've thrown at it - as long as I get my thinking right.

The real question is, do you have enough to pay for the production of the new GTAIV in everything from code to art to sound to marketing to QA? Unity hasn't picked up a reputation for Tripple A games yet, but from what I've seen in the last 4 years or so - it's coming up fast.