Sine Cosine.

Linear movement with sin() and cos(). Numbers between 0 and PI*2 (TWO_PI which is roughly 6.28) are put into these functions and numbers between -1 and 1 are returned. These values are then scaled to produce larger movements.

 
int i = 45;
int j = 225; 
float pos1 = 0; 
float pos2 = 0; 
float pos3 = 0; 
float pos4 = 0;
int sc = 40;

void setup() 
{
  size(200, 200);
  noStroke();
  smooth();
}

void draw() 
{
  background(0);
  
  fill(51);
  rect(60, 60, 80, 80);

  fill(255);
  ellipse(pos1, 36, 32, 32);

  fill(153);
  ellipse(36, pos2, 32, 32);

  fill(255);
  ellipse(pos3, 164, 32, 32);

  fill(153);
  ellipse(164, pos4, 32, 32);

  i += 3;
  j -= 3;

  if(i > 405) {
    i = 45;
    j = 225;
  }

  float ang1 = radians(i); // convert degrees to radians
  float ang2 = radians(j); // convert degrees to radians
  pos1 = width/2 + (sc * cos(ang1));
  pos2 = width/2 + (sc * sin(ang1));
  pos3 = width/2 + (sc * cos(ang2));
  pos4 = width/2 + (sc * sin(ang2));
}