The picture is a series/parallel, not parallel/parallel.0.6 ohms
R(total) = 1 / ((1/r1) + (1/r2) + (1/r3) ... (1/rn))
R(total) = 1 / ((1/4) + (1/4) + (1/4) + (1/4) + (1/6) + (1/6) + (1/6) + (1/6))
R(total) = 1 / (1 + .66666)
R(total) = 0.6
This... Makes no sense to get woofers with different coils.2 dual 4 ohm would be either 1 or 4.
2 dual 6 would either be 6 or 3.
might come out to 4 ohm.
but why bother, just get the same subs with all the right Ohm loads.