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I am planning to publish a book and I am wondering whether to distribute it as ePub3 or ePub4. My concern being that older eBook readers may still be out there and I do not wish to exclude them. I would prefer ePub3 if the older readers are able to read it.

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  • It is unclear what you mean: are you asking if older readers support ePub4 or if new ones support ePub3? – Ian Dec 19 '13 at 22:49
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I believe you are confusing ePub2.0 vs ePub 3.0. If I recall ePub 4 is for Google only. If you are worried about older devices I would focus on validating your .epub file with a 2.0 validation tool.

To answer your question if its backwards, yes, only if it validates across the board:

Example:

<File>
    <Name>foo.epub</Name>
    <ValidationReport elapsedTimeInMillis="230">
        <Version>3.0.0</Version>
        <Valid>yes</Valid>
        <Errors size="0"/>
        <Warnings size="0"/>
    </ValidationReport>
    <ValidationReport elapsedTimeInMillis="177">
        <Version>1.2.0</Version>
        <Valid>yes</Valid>
        <Errors size="0"/>
        <Warnings size="0"/>
    </ValidationReport>
    <ValidationReport elapsedTimeInMillis="149">
        <Version>1.1.0</Version>
        <Valid>yes</Valid>
        <Errors size="0"/>
        <Warnings size="0"/>
    </ValidationReport>
    <ValidationReport elapsedTimeInMillis="156">
        <Version>1.0.5</Version>
        <Valid>yes</Valid>
        <Errors size="0"/>
        <Warnings size="0"/>
    </ValidationReport>
    <ValidationReport elapsedTimeInMillis="201">
        <Version>1.0.4</Version>
        <Valid>yes</Valid>
        <Errors size="0"/>
        <Warnings size="0"/>
    </ValidationReport>
    <ValidationReport elapsedTimeInMillis="182">
        <Version>1.0.3</Version>
        <Valid>yes</Valid>
        <Errors size="0"/>
        <Warnings size="0"/>
    </ValidationReport>
    <ValidationReport elapsedTimeInMillis="193">
        <Version>1.0.2</Version>
        <Valid>yes</Valid>
        <Errors size="0"/>
        <Warnings size="0"/>
    </ValidationReport>
    <ValidationReport elapsedTimeInMillis="208">
        <Version>1.0.1</Version>
        <Valid>yes</Valid>
        <Errors size="0"/>
        <Warnings size="0"/>
    </ValidationReport>
</File>

The reason is because an .epub file can be accepted by some as a 3.0 valid ePub ONLY and it could still fail the ePub 2.0 validation.

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