There are a number of opinions re tyres on the internet, probably well over a billion (hence the jibes you received), so trying to find the correct answer is actually quite difficult.
Having ridden motorcycles for a living (and for over 40 years for fun) when your income depends on things like tyres, you tend to research the subject a little deeper. In times before the internet, (very dark, cold times) research was done in libraries and from information from other professional riders, magazines and tyre companies etc. I still do my own research into all things "motorcycle" and try to use reputable sources like manufacturers' data sheets, well regarded journalists (though they don't usually test worn tyres), my own experience, time-served (usually independant) mechanics, my motorcycling peers (who aren't d*ckheads) etc.
I'm going to assume Radial tyres as the advice for Crossply tyres
is different (yes, I'm that old).
In the last 15 years or so, motorcycle tyres have improved out of all recognition to the tyres we had previously. Therefore, a lot of the old advice is no longer applicable to modern tyres.
TL;DR: In general, you can mix and match Radial motorcycle tyres up to a point...
In your particular case, the Michelin PR and Road series can be mixed and matched (I've done it more than once on my Honda) but, take it easier in the wet as the tread patterns (hence the water clearing abilities) aren't
as optimised when mixed. Though, you'll have much better grip than I had when I was earning a living on matched Crossply tyres! You'll probably be fine with PR4 and Road 6 as long as you don't "Ride like Rossi".
You shouldn't mix and match tyres from different manufacturers or different model classes from the same manufacturer (for example, Michelin Road Series and Michelin Anakee Adventure). Note, "shouldn't" not "must not". If you do, the ride could be "rather interesting", surprising and not what you are used to.
If your only choice is to mix and match different manufacturers or "series", do so but, take it easy when riding (especially in the wet, on gravel and off-road etc.) and change them for a matched set as soon as possible.
Finally, I usually like the Michelin Road series on Touring, Sports Touring, some sports and naked motorcycles. But, on the FJR, I've found that the Michelins square off very quickly and the "feel", even when new, isn't as good, so I run Dunlop RoadSmart IV GT (use GT on the FJR due to its weight - that's a different debate as well
) on my FJR at the moment.