The best replacement for nikon d4 is canon 1dx.

The best alternatives to nikon d4 are rated as following:
★★★★★  canon 1dx
★★  d3x
★★  canon 5d mark iii
★  nikon d800
★  1dx
★  nikon d3s
★  sony a99
  canon d1x
  leica m
  nikon d3x
  canon eos1
  canon 5d mark 3

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Speaking about nikon d4, one probably shouldn't forget about canon 1dx.

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