Impedance Matching Confusion

You’ll be fine since the impedance of the upper and lower section are separated by the internal crossover filters.

If you put 2 pairs of 4 Ohm speakers in parallel you end up with 2 Ohms, but if you bi-wire a single 4 Ohm speaker you still have 4 Ohms.

Whether you bi-wire or single wire a speaker the amplifier sees exactly the same load as before. You are just moving where the jumpers are. On the speaker or on the amp. Electrically identical situation.

It depends on the specific amplifier. +1 What Amplifier? Good stab at it erik_.

The amplifier is a SONY TAN1.

Why not just purchase a pair of bi-wire speaker cables? I use Morrow Audio bi-wire cables. They have one pair of connectors at the amp and a dual pair of connectors for the binding posts on the speaker. Simple!

"The user manual says that it can drive a pair of 4 ohm speakers if only one set of the binding posts are being used"

It would seem that Sony has stated not to use both amp terminals to drive 4 ohm speakers. Why risk potential damage, or at the least I'm guessing per their statement, degrade  the sound. As others have advised just bi-wire , or if you are not presently maybe try quality jumpers