My advice is: Do nothing. You're done! The system you have is fine and adding speakers of any kind will not offer significant improvement but could overload/overheat your amplifier.
Your amp is now working into a 4 ohm load and is therefore delivering the greatest power it can safely. Wiring two 4 ohm speakers in series will produce an 8 ohm load and your amplifier will not be able to deliver the same power without severely clipping the signal.
Wiring two 4 ohm speakers in parallel results in a 2 ohm load which can cause instability and overload/overheat your amplifier.
Wiring two
different speakers in series or parallel will most likely degrade the quality of sound of both and make impedance predictions difficult to ascertain without detailed testing and measurement.
If you wish to experiment with adding speakers I highly recommend driving them with a separate power amp.
http://colomar.com/Shavano/spkr_wiring.html
http://www.bcae1.com/spkrmlti.htm