When the resolution is 72 pixels/inch, the length, width and pixels of A4 paper are 842× 595 respectively.
When the resolution is 120 pixels/inch, the length and width pixels of A4 paper are 2105×1487 respectively;
When the resolution is 150 pixels/inch, the length and width pixels of A4 paper are1754×1240 respectively;
When the resolution is 300 pixels/inch, the length, width and pixels of A4 paper are 3508× 2479 respectively.
Therefore, you can cut the background material according to the standard pixels and aspect ratio.