this seems to work:

  // loop through the 4 oscillators, adding the left to right:

  t_sample l[4];

  t_sample r[4];

  t_sample out[4];

  

  while (n--)

  {

    for (int osc = 3; osc >= 0; osc--)

    {

      l[osc] = *left[osc]++;

      r[osc] = *right[osc]++;

      out[osc] = l[osc] + r[osc];

    }

    for (int osc = 3; osc >= 0; osc--)

    {

      *output[osc]++ = out[osc];

    }

  }