ohms law:
in series, final resistance is sum of the resistances. ie. X+Y=Z
in parallel, final resistance follows the formula (1/X) + (1/Y) = (1/Z)
if you have a 2 dual 4 ohm subs your possible final resistances (as seen by a mono amp) could be 1, 1.6, 4, 10, or 16.
you wanted 4 ohms and...