Reply To: Convert US Letter to A4 ?

These page sizes are entirely different. You can experiment with margin settings that ought to work for both documents (although in both cases you will have a lot of white space at either top/bottom or left/right) by creating a document that has got the width of US Letter and the height of A4: 8.5 inch wide, 29.7 mm high. Draw rectangles on your master page(s) with each of these formats (put them on a non-printing layer) and try to stay within a reasonable distance of all four sides.

Although you can change the 'actual' page size after you worked on your document, it may not even be necessary. You can't (easily?) change the page size while exporting to PDF, but I think it's possible to target a paper size when printing.

The only other option is to design for one size and then put yourself at the mercy of Layout Adjustment when changing to the other size.