The best replacement for 4 speakers is 4x6.

The best alternatives to 4 speakers are rated as following:
★★★★★  4x6
★★  6 1 2

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Speaking about 4 speakers, one probably shouldn't forget about 2 channel amp.

Other search terms related to 4 speakers and their ratings are:
★★★★★ 2 channel amp
2 amps
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Google Trends

Scale is based on the average worldwide traffic of 4 speakers in all years. Click on graph for more.
4 speakers
4x6
6 1 2