Lay out all four pictures in a two-by-two grid on your work surface. For now, leave a few inches between each picture.
Cut a section of builders paper big enough to cover all four pictures by at least few inches on all sides. You may have to tape together two pieces of paper.
Lay the piece of builders paper flat on your work surface. Arrange the four pictures on top of the paper in the same formation they will be in when hung on the wall. Use the level and ruler to make sure they are even and straight. The space between each picture should be equal to the width of the frame.
Holding the first picture in place, trace an outline of it onto the builders paper with a pencil. Repeat for the other three pictures.
Measure the location of the hanging hardware on the back of each picture and transfer this onto the appropriate outline on the builders paper.
Hang the builders paper on the wall where the pictures will be hung. Use the level to make sure it is straight. Tape all four corners to the wall once you are satisfied with the placement.
Insert screws or picture hooks into the wall for each picture at the locations you previously marked on the builders paper.
Tear the builders paper away from the wall.
Hang the pictures on the screws or picture hooks. Use the level to double-check that each picture is straight.