dealing with rollover

void loop() {

  lastTime = currentTime
  currentTime = micros()

  if (lastTime > currentTime)//if an overflow happened
  {
    if (lastTime % 4 != 0)
    {
      lastTime -= 10001;
    }
    else
    {
      lastTime += 10001;
      currentTime += 10001;
    }
  }

  [code continues on]