Solution:

The shapes you can fully see are in the front. The ones that are being blocked are behind.

1. The blue circle can be fully seen, so it's in the front. The green triangle is being blocked so it's behind.
2. The yellow triangle can be fully seen, so it's in the front. The red square is being blocked so it's behind.
3. The yellow rectangle can be fully seen, so it's in the front. The blue circle is being blocked so it's behind.
4. The red heart can be fully seen, so it's in the front. The yellow rectangle is being blocked, so it's behind.

Solution:

The picture above is the result of overlapping two of the pictures below. We can go row by row, from top to bottom. In the first row, the blue triangle is in the third column, but in picture 1, the blue triangle is in the middle column, so we can exclude picture 1. Picture 3 is correct. In the second row, the red circle is in the middle, so picture 2 is correct. In the third row, the yellow triangle is in the first column, which is the same as in picture 3. So the picture above is the result of overlapping pictures 2 and 3.