since we do not know the details of your template and we know nothing about the printers and the specs for generating the PDF (is it a online-service?), it's hard to tell what's the best way to deal with the situation.
What you could do:
1.1 Do the layout in A4, but provide a bleed that is more than what the printers say: 297/210 * the bleed the printers want.
1.2 Export to PDF X/4.
1.3 Place and scale the placed PDF page by page on a prepaired A5 InDesign document.
1.4 Export to PDF for the printers (set the printer's specifications)
2.1 Do like I described in 1.1
2.2 Export to PDF (printer's specifications)
2.3 Ask the printing company, its prepress department, to scale the PDF; they should be able to do that.