The best replacement for html 4 is xhtml.

The best alternatives to html 4 are rated as following:
★★★★★  xhtml
★★★  html 5

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Speaking about html 4, one probably shouldn't forget about css3.

Other search terms related to html 4 and their ratings are:
★★★★★ css3
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Google Trends

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html 4
xhtml
html 5