Hello ,
The 4gb version of the 270X isn't worthwhile , at its price you're better off picking up the R9 280 which has 3gb of Vram and much more capable.
The 4gb is a marketing scam , its originally intended to be able to run 4k resolution but since the card isn't quite capable of playing at 4K resolution then the 4gb of vram seems a waste of money.
More Vram is needed in very taxing games with very open environment or in higher resolutions like 1440p however 2gb should be fine for the kind of details you're be gaming on with the 270x anyways.
Also the 4gb of Vram won't come in handy as usually the card will run out of power before using all of its available VRAM anyways so the 2gb version should be more than enough .
The only time I'd pick up a 4gb version if I was going to crossfire and then play on 1440p or so but then a higher/better card will be more suited to that role , also gaming on multiple monitors fall in that category.
In conclusion : if you have the money to pick up the 4gb version of the card then you're far better off buying the R9 280 which is a better card. The 2gb version will do you fine in most cases