You don't have to run in parallel, you can run in series.
This will make your load 8 ohms instead of 4.
Your amp can run this just fine with no side effects.
Run the + of the amp to the + of the front speaker,
then the - of the front speaker to the + of the rear speaker,
then the - of the rear speaker to the - of the amp.
Downsides to this are;
1) Only half the power getting to each speaker,
2) no fade control,
3) SQ isn't that great (Still better than stock sound though).
