I had the SL600 decades ago, but only recall each speaker with a single pair of terminals. Searching on the web, I found that the original SL600 has a pair of terminals and the newer SL600si version has bi-wire terminals (4). You must have the SL600si model.
You have a choice to either bi-wire or use jumpers. Run the speakers bi-wired by attaching two sets of speaker cables to the same amplifier output and running both cables to the speaker (connect one set to bass terminals and the other set to HF terminals). Audiophiles have long debated about whether there is any benefit to bi-wiring. If you happen to have extra speaker cables, you may as well put them to use.
Or you can run one set of speaker cable to the speaker and jumper the terminals (red-to-red, and black-to-black). The problem with jumpers is that the banana terminals are flush and you can't easily jumper them like the type of terminal commonly used today. @dekay linked to a jumper solution that uses stackable banana plugs.
