Draw 4 suitably placed  rectangles or a shape at the end of draw so it's on top of whatever you want to appear behind?  That's the approach I've used in the past; though there are potential issues depending on the rendering mode you're using.

Or breakShape which would allow for more diverse clipping shapes...

1. void setup() {
2.   size(600,400);
3.   smooth();
4. }
5. 
   
6. void draw() {
7.   background(0);
8. 
   
9.   fill(0,255,0);
10.   stroke(255,0,0);
11.   strokeWeight(10);
12.   for (int i=1; i<16; i++) {
13.     ellipse(i*(width/16),i*(height/16),30,30);
14.   }
15. 
    
16.   fill(0);
17.   noStroke();
18.   beginShape();
19.   vertex(0,0);
20.   vertex(width,0);
21.   vertex(width,height);
22.   vertex(0,height);
23.   breakShape();
24.   vertex(mouseX,mouseY);
25.   vertex(mouseX,mouseY+250);
26.   vertex(mouseX+250,mouseY+250);
27.   vertex(mouseX+250,mouseY);
28.   endShape(CLOSE);
29. }

Thanks a lot! I'll try both your methods.