If you are ever likely to need to edit both in the future, the you can create an A5 document, put a pic box (inc bleed) on the master page, then import the a4 document into these pages, reduce them to the correct size. Job’s a good ’un. The only thing is to make sure bleed on the a4 is at least 4.25mm. That way, when you reduce from a4 to a5, you still have 3mm bleed. I usually make it 4.5 for ease, or 6mm on a3. ( I have a client that regularly needs posters in 3 sizes.)
The advantage to doing it this way, is an edit on one size, updates the other, without doing corrections twice over.
One other final thing to make sure is that any small text you have on the a4 can stand the reduction and still be legible. Any 6.5pt footnote text at a4 would be seriously pushing it at a5 (just under 4.6pt).