Managed to figure out a couple of ways to do this.

Move default cube to correct coordinates, scale to correct length on y, select top edge (at y=4, z=2), right click and dissolve that edge to create the wedge.

Move default cube to correct coordinates, scale to correct length on y, select vertex at x=1, y=4, z=2 then shift select vertex at x=1, y=4, z=0, right click and merge vertices at last. Repeat for vertexes at x=1, y=4, z=2 and x=1, y=4, z=0.
Hope I got my vertex coordinates correct in the aboveâ€¦
Both options appear to give a wedge with 6 vertices so I am happy with these solutions.
Must admit that it didnâ€™t cross my mind to do it the way the instructor demonstrated. Not sure if that is a good thing or not
I am guessing that there are loads of other ways to achieve this as well.