"They" is the overall speaker impedance, not the tweeter impedance. 4 to 8 ohm is probably just an indication of amplifier compatibility.
You can half the value of the capacitor in series with the 8 ohm tweeter to achieve the same crossover frequency as the original.
However, other specifications of the 8 ohm may not be a good match with those of the 4 ohm.
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Very nice pics and interesting pointing out, although the reason of that is unknown (to me).
This is just why in my opinion they cannot be easily modified if you don't know the technology and the design that brought them to that point.
Now it seems that this point is not negligible at all.
Hopefully an expert (and I'm sure that sooner or later he) will be able to direct you on how to best use the tweeter(s) you purchased.
I've been looking at wiring colours shown the OP's photos and comparing them with those shown in the schematic diagram.
In the photo of post 13, a grey wire is connected to the +ve terminal (red wire) of the tweeter which IS as shown in the diagram.
In the photo of post 21, a grey wire is connected to the +ve terminal (yellow wire) of the input terminal block which is NOT shown in the diagram.
Now, if these two grey wires have continuity with each other, wouldn't that short out R1, C1 and RT1?
If that were to be the case, the tweeter would be put under severe duress and it would be no surprise if it sounded dodgy!
Perhaps I'm talking nonsense, but I really like a good mystery!
In the photo of post 13, a grey wire is connected to the +ve terminal (red wire) of the tweeter which IS as shown in the diagram.
In the photo of post 21, a grey wire is connected to the +ve terminal (yellow wire) of the input terminal block which is NOT shown in the diagram.
Now, if these two grey wires have continuity with each other, wouldn't that short out R1, C1 and RT1?
If that were to be the case, the tweeter would be put under severe duress and it would be no surprise if it sounded dodgy!
Perhaps I'm talking nonsense, but I really like a good mystery!
I'm not an expert, but I'm not at all sure about this: we have often seen on the forum that pics can be deceptive and I think this is the case.
I believe the two wires (grey and red) aren't welded together.
I believe the two wires (grey and red) aren't welded together.
You think not?
Could the grey wire actually be soldered to the tweeter chassis?
Attachments
Affirmative. I still think not, but I certainly don't mean to offend you in any way: it's just my opinion.
It could even be that that wire is dangling and has nothing to do with the tweeter, we never know.
For me it's not that important: everyone can be wrong, including me of course.
And you.
Okay, if you care a lot I was wrong
: is it better now?Please, don't take this too seriously, I'm just joking a little now.
Please, don't take this too seriously, I'm just joking a little now.
When I said "If only we knew someone who owns a pair of these speakers and could reveal to us all of the wiring and components!
", THAT was a joke (referring to the OP)! 
So, I double checked my 401s. The woofers have a date stamp of 8/16/90. The wiring pretty much follows the diagram above. The red binding post has a yellow wire and a grey wire attached. The yellow goes to the front woofer and the grey goes to the crossover. Out of the crossover the wire is red and goes to the tweeter. The tweeter has the red and a different grey wire attached to the same terminal (pos). That grey wire goes to a "middle post" on the front woofer. The black wire from the tweeter goes to the black binding post. As you can see from the included picture of the back woofer, there is an unused "middle" terminal. That is where the grey wire attaches on the front woofer. I don't understand either but if you have any more questions I can try to answer.
Thanks for doing that!
So the yellow wire on the terminal block goes straight to the woofer while the grey wire goes direct to the tweeter crossover components.
A different grey wire connects the red terminal of the tweeter to the chassis of the front woofer.
That confirms that the two grey wires are electrically separate!
Yes, apart from the doubling up of the yellow and grey wires from the terminal block, that is in accord with the crossover diagram.
So the yellow wire on the terminal block goes straight to the woofer while the grey wire goes direct to the tweeter crossover components.
A different grey wire connects the red terminal of the tweeter to the chassis of the front woofer.
That confirms that the two grey wires are electrically separate!
Yes, apart from the doubling up of the yellow and grey wires from the terminal block, that is in accord with the crossover diagram.
So, connecting the positive of the tweeter to the woofer basket must necessarily have an electrical reason (which corresponds to an improvement in the SQ?), frankly I can't imagine any other reasons.
My view is that the fact it was done implies a reason which, although still mysterious, appears to me to be "technologically fascinating" since the designer believed in it to the point of increasing the number of operations to be carried out to finalize his project in an industrial context (and therefore in a factory, carried out by a specialized worker) while generally speaking those number of operations are always optimized downwards rather than be unnecessarily increased without any reason (for an obvious reason of time and costs).
My view is that the fact it was done implies a reason which, although still mysterious, appears to me to be "technologically fascinating" since the designer believed in it to the point of increasing the number of operations to be carried out to finalize his project in an industrial context (and therefore in a factory, carried out by a specialized worker) while generally speaking those number of operations are always optimized downwards rather than be unnecessarily increased without any reason (for an obvious reason of time and costs).
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