The 8 ohm jack wants to see a total 8 ohm load, and the 4 ohm jacks want to see a total 4 ohm load. You would connect an 8 ohm speaker or a speaker array whose load totals 8 ohms to that jack. If you have a single 4 ohm speaker or a speaker array summing to that impedance, you would connect it to one of the 4 ohm jacks. You would use both jacks in a situation where you wanted to run two cabs, and the total impedance of the two cabs in parallel should equal four ohms. In parallel, an 8 ohm load plus another 8 ohm load equals 4 ohms, so if you had two 8 ohm cabs you would use the 4 ohm jacks.
