Measure the distance from the floor to the top of your thighs. This is the maximum comfortable distance for the height of your bed. Measure the current height of the top of the bed and subtract the bed height from your maximum height. This is the height for your bed risers.
Cut the 4-by-4 into four equal sections, the length of which equals the height you determined for your risers. When marking the length, use a T-square to keep the lines absolutely straight and level.
Draw a square one-half inch inside the top face of each riser. Use the T-square to keep it precise. If there is any variation off true square on one end or the other, use that face for the top, so the level face can sit on the floor.
Plunge your router into the top face of the riser on the line you drew. Follow the line around the top of the riser until you've made a square. Repeat the process, cutting away the next section inside the square until an even surface is on the top of the riser surrounded by the half-inch strip on the edges. Repeat for each riser.
Sand all the surfaces smooth. With help, lift the bed so its feet sit inside the cutout area on the tops of each riser.