The best replacement for 100 400 is 70 200.

The best alternatives to 100 400 are rated as following:
★★★★★  70 200
★★★  400
★★  70 300
★★  sigma 50 500
★★  50 500
★★  sigma 80 400
★★  sigma 150 500
★  nikon 80 400
★  150 500
★  canon 400
★  28 300
  300 f4
  sigma 100 300
  sigma 120 400

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Speaking about 100 400, one probably shouldn't forget about teleconverter.

Other search terms related to 100 400 and their ratings are:
★★★★★ teleconverter
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100 400
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