Basic vector math addition

I feel like I should know this because we went over it in calculus 3 but Im having hardest time getting my vector math to work.

Basically I need to draw a box in the editor based off of this information

public int boundaryLength = 10;

public int boundaryWidth = 10;

public GameObject boundaryStart; // where to start drawing from

I then try to draw the box with the following code

void  OnDrawGizmos ()
{   
    //draw a greenbox around the boundary area
    if(visualizeGrid == true)
    {
        //draw a a green boundary box
        Vector3 point_1 = boundaryStart.transform.position;
        Vector3 point_2 = new Vector3(boundaryStart.transform.position.x + boundaryLength,0,0);
        Vector3 point_3 = new Vector3(boundaryStart.transform.position.x + boundaryLength, 0, boundaryStart.transform.position.z + boundaryWidth);
        Vector3 point_4 = new Vector3(0, 0,  boundaryStart.transform.position.z + boundaryWidth);   

        Gizmos.color = Color.green;
        Gizmos.DrawLine(point_1, point_2);
        Gizmos.DrawLine(point_2, point_3);
        Gizmos.DrawLine(point_3, point_4);
        Gizmos.DrawLine(point_4, point_1);      
    }
    else
    {
        return; //if visualize gride is turned of, exit function    
    }
}

This to me seems like it should draw, but I get this weird quadrilateral that is a half box half...not box

Maby my math is wrong but i cant seem to see where

Vector3 point_4 = new Vector3(0, 0, boundaryStart.transform.position.z + boundaryWidth);

Your x,y for this is always zero. transform.position is in world coords. I think you want some thing more like:

Vector3 point_4 = new Vector3(boundaryStart.transform.position.x, 0, boundaryStart.transform.position.z + boundaryWidth);

In other words, your 'local origin' will be transform.position.x and z, and the offsets will be position.x + length and .z + width.