What Charlski said, except that ID4 export to epub is just a starting point.
It will need some tweaking in order to become an epub worthy of the name. The biggest fault is that all the body of the book will be in a single .xhtml file within the epub, which will break some readers.
I know, because our small publishing house is now involved in converting our print books (imposed in InDesign) to epub, and I get the task of tweaking the output of ID4 so it will meet our standards.
Still, the following steps are orders of magnitude better than trying to convert from PDF, which is probably the worst possible choice of starting material (due to the diametrically opposed goals of each format).
Starting from the ID4 exported epub, you will need to:
1) expand the epub to a working directory
2) clean up the styles, and make sure that chapter headers are recognizable (i.e. <h1> tag instead of something like <p class="Heading-1"... or such.
3) Add chapter breaks (to break up the single book body file into manageable chunks, via something like sigil)
4) Adjust the CSS file to reasonable values.
5) Re-zip the working directory to an epub file
6) import the new .epub file into sigil, or some equivalent which AFIK does not yet exist.
7) make necessary modifications to text, and metadata, and save.
9) you're done!
Sounds like a lot of work, doesn't it? and I have probably left out a few critical details at that. But really, it isn't that bad. Less than an hour to come up with a usable epub that will pass epubcheck.