If you were able to play battlefield 3 on high settings without lagging, then you should be able to play battlefield 4 just fine on medium settings. If you played the beta and did experience any lag or fps drops, it was probably a driver issue as even people with high end systems experienced these issues. Hopefully the final release will be much smoother than the beta.

I don't think BF4 will outsell GTA V, The reason is GTA V has a massive fanbase and its genre appeals to a lot of people (who doesn't love a free roam world where you could do almost anything?) and GTA V also had a massive advertising campaign so that even people who haven't played GTA games before wanted it.

But I'm sure BF4 would outsell COD if the people finally realized that COD ghosts is still in the previous gen along with Modern warfare 3. Although, there is a possibility COD ghosts could still outsell BF4 given that the COD fanbase still hasn't gotten sick of playing the same game every year.

In the end, it all boils down to the hype created by the developers/publishers prior to game release. And the trailers and beta of BF4 have created a substantial amount of hype but not enough to beat GTA V in sales.