The best replacement for d4 is d800.

The best alternatives to d4 are rated as following:
★★★★★  d800
★★★★  d3
★★★★  e4
★★★  d3s
★★★  1dx
★★★  canon 1dx
★★★  d4s
★★  super hd
★★  d5
★★  d3x
★★  d6
★★  d600
★★  nikon d800
★  d610
★  dm5
★  5d mark iii
★  d3 honda accord
★  esf
★  df
★  vvti
★  oxyelite
★  d700
★  d4r
★  d800 low light
★  oxyelite pro
★  e4 chess
★  d12
  b44
  d3 driving
  d3 honda
  super hd cellucor
  d3 car
  d10
  canon 5d mark iii

Your alternative for d4 is not listed? Suggest!
Speaking about d4, one probably shouldn't forget about d3x.

Other search terms related to d4 and their ratings are:
★★★★★ d3x
You have in mind a companion for d4 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of d4 in all years. Click on graph for more.
d4
d800
d3