Not quite Big 4, but maybe Big 3+1 or even Big 2+2. Delpo has been on the verge of taking Andy Murray's former place as the "best of the rest." It seems that David Ferrer has taken that spot, so the question is whether Delpo can edge past him. I think he can, but its questionable whether he'll be as consistent or have the stamina.
If Rafa, Novak, and Andy slip at all, Delpo is the player who can take advantage and win a Slam. If I were to guess, I'd say that in the next two years--2014-15--we'll see Rafa win 2-4 Slams, Novak win 2-3, Andy 1-2, Delpo 1-2, and maybe a new Slam winner the mix. I do think he'll win at least one, though.
NADAL2005RG said:
If Murray ever regains his level, it'll be the strongest top 4 of the 21st Century, now that Del Potro is consistent, Nadal is healthy, Djokovic is the ideal #2.
I'm going to go out on a limb here and disagree with you. The Big Four of 2012 was much stronger, at least until Rafa went down. But dial back to the tour going into Wimbledon - you had Rafa and Djokovic battling it out, you have Federer surging, you have Andy waiting in the wings and then winning at the Olympics. There is no way the current (possible) "Big Four" will equal that, unless you think Delpo can be as good as Roger was last year.