title_body: Is ePub3 backward compatible?
upvoted_answer: 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:
  
  &lt;File&gt;
      &lt;Name&gt;foo.epub&lt;/Name&gt;
      &lt;ValidationReport elapsedTimeInMillis="230"&gt;
          &lt;Version&gt;3.0.0&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="177"&gt;
          &lt;Version&gt;1.2.0&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="149"&gt;
          &lt;Version&gt;1.1.0&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="156"&gt;
          &lt;Version&gt;1.0.5&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="201"&gt;
          &lt;Version&gt;1.0.4&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="182"&gt;
          &lt;Version&gt;1.0.3&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="193"&gt;
          &lt;Version&gt;1.0.2&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
      &lt;ValidationReport elapsedTimeInMillis="208"&gt;
          &lt;Version&gt;1.0.1&lt;/Version&gt;
          &lt;Valid&gt;yes&lt;/Valid&gt;
          &lt;Errors size="0"/&gt;
          &lt;Warnings size="0"/&gt;
      &lt;/ValidationReport&gt;
  &lt;/File&gt;
  
  
  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.