The OpenIV modding team is bringing Liberty City to GTA V on PC, but you have to own both games.
You'll soon be able to visit GTA IV's Liberty City in GTA V, thanks to the magic of modding. In a blog post, the OpenIV mod team says that it has been working on this project since the PC version of GTA V was released in 2015.
The mod will bring Liberty City in as an added area to explore; it will not replace Los Santos. Instead, it will be across the ocean, and you'll be able to fly there. Here's a teaser trailer for the mod.
If you want to check out the mod when it becomes available, there is a bit of a catch. To stay away from any possible legal issues, the mod won't actually put Liberty City into GTA V. In order to play it, you'll need to own both GTA v and GTA IV on PC. Then, you'll download a tool from the modders that will extract Liberty City from GTA IV and create a custom DLC that you can add into GTA V.
There's no release date for the mod as yet, but you can keep an eye on the mod team's official site for updates.