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];
}
}