If your amp is stable at 2 ohm mono, you can parallel 2 4 ohm subwoofers. If it stable at 2 ohm stereo, you can parallel 2 4 ohm subs on each left and right channel.
The pros and cons of parallel verses series is in a parallel load, the impedence goes down where the amp can deliver more power. In series, the impedence goes up cutting down the power of the amp. (pros) amp delivers more power to driver. (cons) the amp gets very hot and the clarity diminishes. The higher the impedence load, the cleaner the signal to driver but you lose the full power potential of amp.
You can take 4 4 ohm subs and do a combination of parallel and series and still create a 4 ohm load on an amplifier.
High level inputs means that you can use the + and - of a deck powered speaker out and use that to feed an amp at high level to give it input signal, where low level inputs will be pre-outs on your head unit usually rated at 2 to 4 volts on the pre-amp section. I hope I didnt confuse you, speaker load is very critical to the performance of an amplifier and if you go below the level of its rated stable load, you can run an amp hot and cause it to shut down.