Try them all and use the ones that sound best to you. There is no right or wrong.
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Welcome to Audiogon. You are going to find that you will get a variety of opinions on any topic or question you post. I consider that a good thing, but as you can see you already received two different answers that could have you going two different directions. Here is my advice: Read the Prologue manual (hard copy or online) to determine whether it specifically says you should not connect 4ohm speakers to anything other than the 4ohm posts. If it doesn't tell you to expressly avoid such connections, then I would try them all and see what sounds best to you. This is a hobby. As such, you should try different options when they are available unless they are patently dangerous, will void the warranty, or you're not sure if it will damage the equipment. I would be surprised if trying your speakers on all the optional speaker posts will hurt either your speakers or your Prologue.
Speakers ratings are always approximative, just look at any measured impedance curve. They give a rough idea of how hard they might be to drive. I doubt many amp's 4 and 8 ohm hook-ups are exact either. The obvious thing is to start with 4 to 4, let the amp run in, get familiar with the sound, then change to 8 to see what differences you notice, and whether they're better or worse.