Have a sheet of plywood cut to the size you want for your divider. Most hardware stores will do this for you. Have your 2-by-4 plank cut to the same length as the bottom of the plywood sheet.
Sand down the plywood sheet with the sandpaper, particularly along the edges. This is to smooth out the sharp edges, which could cause cuts, and help prevent splinters.
Screw a roller wheel to each corner of one side of the 2-by-4 plank on the 4-inch side. The plank will serve as the base of your divider. The roller wheels will be attached to square-shaped screw plates with four screw holes in each corner. Use a power drill to guide wood screws through the holes in the screw plates and secure the roller wheels to the plank.
Set the plank on its wheels and test it to make sure it rolls around properly. It should resemble a long, narrow skateboard.
Have your partners center the plywood sheet over the plank. Make sure it is perfectly centered and standing straight up, perpendicular to the plank. This is essentially what your bare divider will look like.
Attach the plywood sheet to the plank with the mounting brackets. The brackets are pieces of metal bent at 90-degree angles with screw holes on each end to connect the sheet to the plank. Place a bracket at each end of the sheet on each end and one in the center. Repeat the process with the other side of the sheet.
Test your divider by rolling it around and making sure it isn't too wobbly. Attach more mounting brackets if it seems shaky at the bottom.