It runs on some people's laptops (not even gaming laptops) at reasonable quality settings and resolutions like 1080p. That makes it easier than most to run at high resolutions on a desktop gaming PC. It's caused by two things. Firstly it's obviously well optimized, and secondly, sometimes it looks like crap for a 2015 game. Which is because the base game is from... 2013? And for consoles. The best example of it is during the tutorial, when you get in a car that looks like things haven't moved on at all since GTA: San Andreas. Overall, the game is a mixed bag of great high poly models, average models, and terrible eyesore models.
On my PC it's the easiest game of late to run at 4K. It runs smoothly on GTX 980 SLI without sweating the GPUs. But it has some very strange framedropping happening occasionally which I can't pinpoint but would assume is the content streaming tech working (badly). In terms of system resources it might be VRAM running out and having to be repopulated, since the GTX980 is light on VRAM (only 4 gigs). The other hardware should be fine (i7 5930k at 4,2 GHz, 32 gigs of DDR4, 1TB SSD with more than a third of it empty).
The 4K experience in GTA V isn't as incredible as all the hype makes it out to be. It's nice for detail in certain scenarios (cutscenes with closeups of people, flying, offroading, looking out in the distance). Otherwise 4K works much better in open world environments with lush foliage and high details, like the latest Dragon Age, Skyrim, Tomb Raider and so on. 1080p just can't resolve the details of foliage and that makes the 4K experience so amazing. GTA on the other hand is mostly cityscapes, desert and ocean and while it's nice at 4K, it's not mindblowing, because it's old hat by 2015.
I have a couple of screenshots on my onedrive if you want to have a look. It's at 4K almost maxed settings - yes, even the Ultra settings which some people have missed. IIRC one of the advanced sliders didn't go all the way up because the VRAM-meter very helpfully prevented it. Anyway, compare the graphical fidelity to Inquisition for example, and judge for yourself.