If you want to run both pairs of mains simultaneously, then splitting the pre-outs via a Y-cable is probably the best solution. Keep in mind that when you split a signal like that, each branch of the split will receive a weaker signal. This isn't usually a problem since you can just turn up the volume.
As to running the speakers in series, it would actually double the ohm rating (totalling 8 Ohms), not halve it. Running speakers in series is not recommended because as the signal passes from the first speaker to the second, the resistance of the first speaker changes the signal. So the signal going into the first speaker in the line won't be identical to the signal entering the second speaker.
A series connection looks like this:
AMP <====> Speaker 1 <====> Speaker 2
What you'd want to do is run the speakers in parallel. This prevents the two speakers from interacting, but it does halve the resistance (effective 2 Ohm resistance).
A parallel connection looks like this:
Speaker 1 <====> AMP <====> Speaker 2
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