The different taps have different source impedances. Â The 4 ohm tap has the lowest impedance of the amp itself in order to minimize the frequency response changes from the interaction with the low impedance of the speaker. Â This tap would also lower the frequency response changes of higher impedance speakers too, but those response changes are much smaller and of less consequence. Â The tradeoff of using the 4 ohm tap is that less power can be delivered to your speakers. Â
Use what sounds best.  It might be the case that you will like the frequency response changes wrought by using the 16 ohm tap with your 4 ohm rated speaker.   This is not usually the case, but it doesn’t hurt to give it a try.
4 8 or 16 ohms.
Hi group I just purchased the Audio Research REF 750s mono amps. On the back for the speaker terminals are 3 sets of binding posts. 4 8 and 16 ohms. I will be using B&W Matrix 800 speakers. I believe they are 8 ohms but not 💯 sure. I looked in the manual but can’t find anything about ohms. Someone said to me it does not matter what ohms I connect my speakers to. He said just use the ones (ohms) that sound best. Does that seem right to do ? I don’t want to damage my speakers or amps.  BTW if it matters I will be using an Audio Research REF 6SE pre amp.Â