I see their water also looks great.
I bet they've learned a lot from the graphics mods made by fans for Gta4.

I see they've also added depth of field to the engine and the car models have shading and poly count, almost comparable to the ones seen in the gta 4 fan made high quality cars.

I'm not sure but I think here we are shown dynamic volumetric lighting in action, which is also a new engine feature that didn't exist in gta 4.

My guess is that the gta5 engine, is at least as well optimized and at least as modern as the max payne 3 version of it, if not more!
The gta4 engine has evolved a lot after they used it for Red Dead Redemption and for MaxPayne 3 and now for gta5, they improved it a lot.
They also probably aim to make Gta5 a much smoother experience on pc than Gta4 was at release date and considering they use the same engine as gta4 but much more updated and fixed, I don't see why they won't take their sweet time to avoid all of their previous mistakes and prevent the need to patch the shit out of the game afterwards.