Basically, if you go on G2A and create an account, then search GTA IV Complete Edition, (When I search there is the North America Key Version or Global), so as long as it is compatible for where you live, it depends on the price. Search the game and find the compatible version, and follow things through. One thing you may have to do is select the area where you live, and that will add any tax that is required by law.
If you live in North America, you can follow this link:
https://www.g2a.com/grand-theft-auto-iv-complete-edition-steam-cd-key-na.html
This is one of the region locked versions of it, so it will only work in North America, if you don't live in North America, I don't think you can get it, as when searching on G2A, there are no other versions for it, other than Xbox 360. I am guessing you'll be getting the Steam key, which means you'd have to purchase it elsewhere.