All the speakers, amp and head unit are 4 ohms. Can I connect each side separately in serial or could it damage the amp?
Cheers ...
Are you using the stock H-D speakers in the rear pods?
If you are using the stock rear pod speakers, they are 2-ohm nominal impedance. They can be hooked in series with the saddlebag speakers for a nominal load on the BT-4180 rear amp of 6 ohms. The 4180 should have no issues driving the slight increase in impedance on the rear amp, although the power output may suffer. As far as I am aware, there are no gain adjustments on the 4180 amp so the only ability to balance the amp output rests on processing capabilities of the head unit.
An option for you would be to add a 2 ohm stable amp in the Tour Pak to drive the rear pods, but in all honesty, the stock speakers will always be the weak point. They just aren't very good. Not to add insult to injury, but there also aren't any high quality speakers on the market that will replace the stock rear pod drivers without some sginificant work.
You could drop a set of Polk Audio DXi-650 in the pods (will require new holes drilled) - then wire the Kicker saddlebag speakers in parallel with the new DXi-650 pods. This would drop total load on the rear amp to 2-ohm (the 4180 is 2ohm stable) and provide a slight increase in power output. Again, balancing the system would rest on capability of the head unit.
I wouldn't recommend trying to do a sub setup in the TP. Might sound great while parked but most of the low frequency will be lost at speed. Not worth the cost of $$ and space in my opinion.
Hope this helps ...