I don't think the issue is if an AMD platform can power quad GPU, but how effectively the platform scales feeding four GPUs. Can't say I've seen anyone saying that an AMD FX platform can't do quad Crossfire (or SLI). Point them out to me, and let's have some fun.
As for feeding the GPUs in comparison with Intel platforms, I don't think the numbers lie. The highest placing in the four most intensive quad-
GPU benchmarks for an AMD platform is 31st ( an FX-9590/990FX) for 3DMark Fire Strike Ultra using four R9 295X2's). The preceding 30 places are obviously Intel. The next best result is your 45th place in 3DMark11 Extreme, followed by 152nd place ( Phenom II X6 1090T) in Unigine Heaven Xtreme Preset, and 279th place (also a 1090T) in 3DMark Vantage Performance Preset.
It makes for pretty telling reading considering the number of 2500K/3570K/4690K 4-core systems putting up better quad-GPU benches than many (nominally) 8 core FX's.