This is very insightful. Imagine how hard you could run that one die with a thermal resistance of only 3 ºC/W!. There is no doubt now that Cree is king of the led world.
A single die won't have a thermal resistance of 3 C/W. The thermal resistance is the rating when all 4 dies are powered. A single die will still have a thermal resistance of probably 8 C/W.
This and the Seoul P7 get a lower thermal resistance because each watt of input power is now spread over 4 dies, and not just one. The per-die thermal resistance is the same as it was before. There have been no improvements in thermal resistance of the individual dies...it's just a multiplicity thing that's changing the spec.
Consider this - An XR-E has a thermal resistance of 8C/W. If you run it at 350mA, Vf = 3.2V, holding the heat sink at 25C, then Tj will be (3.2*0.35*8)+25 = 34C
Now run this guy with all 4 dies at 350mA each. Tj of the dies will be (3.2*0.35*4*3)+25 = 38C.
At 1A, Vf=3.6V, the XR-E Tj will be 54C, but the quad's Tj at 1A/die would be 68C.
Even though the thermal resistance is lower, running at the same per-die current results in a higher Tj. That's probably why these are only rated for 700mA/die - with all 4 dies powered, the per-die thermal resistance is actually HIGHER than with an XR-E.
Why?
It's because the packaging thermal resistance hasn't improved at all. The total junction->heat sink thermal resistance is the sum of the junction->attach thermal resistance, plus the package thermal resistance. By putting 4 dies in the same package, you've reduced the junction->attach thermal resistance by 4 (assuming the same total input power to the device), but the package thermal resistance hasn't changed at all.
The 8C/W of the XR-E is likely 6ish C/W of junction->attach (the die), plus 2C/W for the package. Stick 4 of those same dies into one package, now junction->attach is 1/4 (so 1.5C/W). Improve the package a bit to reduce it, and now you're down to 3C/W, and you haven't done a thing to the thermal resistance of the individual dies. Your only R&D has gone into packaging - after all, why mess around with your dies when you don't have to?
It was the same with the Seoul P7. The thermal resistance is significantly lower than with the P4, but Seoul couldn't do anything to the dies, since they buy them from Cree - it's simply a result of sticking 4 of them in one package, and looking at the total power input to the package being spread out into 4 dies, instead of one.