Usually don't have time for such complicated hypothetical comparisons, but let's give this one a go.
Assuming that we're only looking at hypothetical lineups where these 4 are the strongest 4 on their teams, and not the actual team lineups they played on, because that skews things, and you have to consider things like Gilly's role, as Spark mentioned, or Kallis' bowling, etc.. So nah, let's just go with these being your top 4 batsmen, and everyone else on the team is a weaker batsman, and not considering any non-batting considerations for any player. Also, not going for longevity in this comparison, just output quality in case you're thinking things don't exactly match the consensus of Tendulkar >>>>> everyone, getting that out of the way.
Given that, I think the 4 Aussies are very clearly the strongest, followed by the Indians, then the South Africans.
Hayden > Sehwag => G. Smith
Ponting < Tendulkar = Kallis
Gilchrist < Dravid > DeVilliers
Waugh >>> Laxman << Amla
Indian 4 could possibly have been competitive or possibly better than Australians, but for me Laxman is very clearly the odd man out in this whole comparison and a step down in quality from any of the others. Clutch at times, but I don't think has anywhere near the consistently excellent career runs that the rest have under their belts. And although India have rarely lacked for quality bats in the last 4 decades or so, during the "Sachin-Dravid-Sehwag" era there really is no 4th who isn't a major step down.
In practical terms there is a caveat to Australia too though. Waugh didn't really play in the other 3s true combined hayday of the mid 2000s, so there's that. Also to note the SA batting lineup was truly dire outside of this "big 4" during their era ( which also didn't exactly neatly line up for all of them, the lynchpin Kallis only played half of his matches with deVilliers, for instance ).