The best replacement for ddr4 is ddr3.

The best alternatives to ddr4 are rated as following:
★★★★★  ddr3
★★★★  gddr5
★★★  lpddr4
★★  ddr3l
★★  ddr5
★★  hmc
★  ddr2
★  ddr3 ram
★  lpddr3
  xdr2
  ddr4

Your alternative for ddr4 is not listed? Suggest!
Speaking about ddr4, one probably shouldn't forget about haswell.

Other search terms related to ddr4 and their ratings are:
★★★★★ haswell
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Google Trends

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ddr4
ddr3
gddr5