What is your target resolution and target fps rather quality? If you planning to play in low res than 1920x1080 with mid setting and all you need is 30-60 fps then you can play with 2gb gpu. However if you want to make your game experience smooth for a long run say 3yrs in that case even you play in 1080 to 720p buy a 4gb gpu. Game needs more vram to load lot of data to render the game properly. Just keep in mind a 4gb gpu will not provide you a good fps jump than a 2gb same model but your experience will vary a lot in near future.