This is roughly an inch larger than a booklet for a CD. It's pretty small. Depending upon your audience for this size of brochure, it can easily get lost in the briefcase of the recipients.
If you haven't already, I would encourage you to talk with whoever is printing this to see what the cost is jumping to A3. I mean, I can squeeze a brochure onto a landscape letter-sized sheet, but the cost to go to Letter-Extra is incredibly marginal. The cost to push it onto a Tabloid-sized sheet is in the neighborhood of pennies if the print run is 500 pieces.
That said, if it were me, I would simply use a full size page of half an A4. Let ID put the bleed, color bars, etc., into the PDF. It will be larger than your required A4 sheet. But I would then take the PDF and print to the Adobe PDF print driver and opt to have Acrobat set up to use an A4 sheet and scale it to fit. You'll get maximum print area without all the back and forth adjustments in order for ID to get all that onto an A4 sheet. Just a thought.
Take care, Mike