Im not sure if this is the most logical way but you could use either cull index or cull pattern. Cull pattern you set multiple booleans where as true says take the point and false says ignore it. The other is cull index where you simply set multiple integers and type the number of the list items you dont want. The problem with this is its not so parametric as you have to change the numbers manually if you change the order of the list or if you change the amount of segments or points. I'm sure there is a better way.
Here's a definition that uses a cluster I wrote to measure the interior angles of a polygon. It then isolates the points with the smallest angles, based on a threshold value you set.
You could also make a circle big enough to contain the shape and then only get the four points whose xy coordinates are closest to the circle. This should generally get just the four corner points if you have a roughly rectangular polygon.