Set report (RDLC) as always landscape printing and A4

Double click on your report (ex: report1.rdlc) in Solution Explorer then right click on the report page and select Report Properties and you can select your prefer paper size and orientation as shown below

As A4 size is 8.3 inches × 11.7 inches(Portrait).

So you need to set it to

report properties>Layout>Set page width and height.

Or you can manually print the report.

Look at this tutorial

Printing RDLC Report manually

You can certainly avoid the print dialog and print directly, also you can specify page size and print orientation in code. We do it in our projects but the code is a bit of muddle plus it's in VB so I won't post a chunk of it here.

MSDN has a page titled Walkthrough: Printing a Local Report without Preview. This code has some XML to set the page size:

      "  <PageWidth>8.5in</PageWidth>" + _
      "  <PageHeight>11in</PageHeight>" + _

Change this to 29.7cm & 21cm for A4 Landscape.