There are many ways of doing this but a simple way is to use the Table icon. Click on the table icon. Draw a box the size of the page you want ie A4. Set the number of rows and columns to 2. this will give you the basic layout for 4 postcard size boxes. Then copy and paste your poster into a box. Once this is done and sized correctly to the box copy and paste it into the remaining boxes. Hope that this helps.