I would have made the sketch for the cut in the centre of the cube (create a midplane if it's not centered around the origin planes), and then use extrude with cut all to one side only. If the cube is not centered around the origin planes, then make a midplane for the other direction of the cube (perpendicular to the first midplane), and make an axis in the intersection between those two planes. From there you have a couple of options:
- Use Circular pattern around the centre axis, select 4 as the number of instances. (see Cube attachment)
- Use Circular pattern around the centre axis, select 2 as the number of instances, and 90deg as the angle. Apply two mirrors to complete the design. (see Cube 2 attachment)
You can see the difference between the two options/files:

