The best replacement for dr-40 is dr-100.

The best alternatives to dr-40 are rated as following:
★★★★★  dr-100
★★★★  h4n
★★★  dr-100mkii
★★★  dr-60d
★★  zoom h2n
★★  sony pcm-m10
★★  zoom h4n
★  dr-05
★  dr-100 mkii
★  h2n
★  dr-07mkii
  dr-07

Your alternative for dr-40 is not listed? Suggest!
Speaking about dr-40, one probably shouldn't forget about rode videomic.

Other search terms related to dr-40 and their ratings are:
★★★★★ rode videomic
★★ rode ntg 2
You have in mind a companion for dr-40 and it is not yet here? Propose!



Google Trends

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dr-40
dr-100
h4n