Tweeter quest

Nick, forget it -- unless pmc themselves help you out with indispensable info on their speaker "voicing", the crossover schematics and the characteristics of the tweet you're now using.
In order to replace the tweet with a ribbon you need to mate it seamlessly with the other drivers.
This is difficult to do in yr present application anyway as ribbons are best cut higher than their cone, dome, etc counterparts.
You don;t seem to have been designing & building speakers as a hobby or otherwise. Ergo, let things be as they now stand UNLESS you wish to take on a new hobby and use your present speakers for experimenting. IMO you'll need lots of help in the beginning -- and do some reading of your own.

The "best" ribbon driver doesn't exist. "Best" is matter of applications. Some ribbons look better on paper than others; Alian ribbons are amongst the latter

Measure the existing audax. 4kHz isn't a bad xover point; I would choose the cheapest (i.e. Chinese) true ribbon that has the right nominal sensitivity (i.e. ~equivalent to the audax -- or higher).
Can you find out the hi pass particulars and if there's any "taming" (notches, paddings and the like) taking place?

Fo a much used and tried model, there's the Raven (1 or 2, both expensive).

The problem you may have is the ribbon not matching sonically (the perception of "speed" is often the problem) with the next drive unit. Phase anomalies can raise their ugly head... that's why I recommend cheap first.
Also beware of high wattage; ~30 is a safe max & you're best starting out with a 24db LR cut.

The PMC looks very well thought out as is, you see. What are the other drivers, BTW? Volts?