PShape shape = createShape();
shape.beginShape(QUAD);
shape.noStroke();
shape.fill(255, 0, 0, 100);
PVector pos = new PVector();
shape.vertex(pos.x, pos.y);
shape.vertex(pos.x + 4, pos.y);
shape.vertex(pos.x + 4, pos.y + 4);
shape.vertex(pos.x, pos.y + 4);
shape.endShape();
shapeGroup.addChild(shape);