If you wire two 8 ohm speakers in series you end up with 16 ohms total load.
If you wire two 8 ohm speakers in parallel you end up with a 4 ohm load.

If you wire four 8 ohm speakers in Series/Parallel you end up with an 8 ohm load. It works this way.
Two 8 ohm speakers in parallel = 4 ohms and you have two sets of parallel speakers so you have two 4 ohm loads. Those 4 ohm loads are now added together in series and so you are back to an 8 ohm total load.

Two 100uf/350v caps in series = 50uf/700v.
Two 20uf/500v caps in parallel = 40uf/500v