The best replacement for 4 ohm is 8 ohm.

The best alternatives to 4 ohm are rated as following:
★★★★★  8 ohm
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★★★★  2 ohm
★★★  8 ohm subwoofer
★★  16 ohm
★★  2 ohm sub
★★  1 ohm
★★  6 ohm
★  2 ohm speakers
★  2 ohm subwoofer
★  2 ohm subs
  8 ohm bass
  10 ohm speakers
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  2 ohm subwoofers
  4 ohm
  dual 2 ohm
  8
  dual 2
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  2 ohm better
  2
  2 ohm speaker
  8ohm speakers
  single 4 ohm

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Speaking about 4 ohm, one probably shouldn't forget about 8 ohm.

Other search terms related to 4 ohm and their ratings are:
★★★★★ 8 ohm
★★★★ 8 ohm speakers
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