I have 4-divs

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[1] [2]
[3] [4]
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What I want is to resize each <div> to 300px whenever I use them on smartphones and they will re-arrange automatically this way.

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[1]
[2]
[3]
[4]
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I was able to do to the auto-resize using "%" but I cannot limit to 300px and it doesn't go to the next line.

I can't find an accurate solution using Google.

I appreciate any help. Thanks