A warning right away: no matter how positive the responses to this are - I am not sure I will be able to make the time needed to actually run it. And if I do, it'll take months rather than weeks to prepare.

Now to the potentially odd edition I would choose. I know there are quite some oddities to the rules, but frankly, I do not care that much about mechanics. Actually, the overload of mechanics is what I don't like about 5th - there is apparently a rule for everything...
Also, when I will think about a saga, I will probably have some more or less concrete ideas about characters I would need for the story to be told. Not saying I'd go all the way to pre generated, but there might be constraints and I might be looking for particular concepts rather than completely free applications.

Would there be interest in a 4th edition saga that focuses on telling a story and potentially quite some hand waving when it comes to rules?

Maybe some examples are in order.
Even the mystery houses of 5th core + houses of hermes are, from my perspective too rule heavy. If you put mysteries in fixed rules, where's the mystery? On the flip side, you might get to play a Bjornaer with a magical heartbeast, if that is what drives the story forward.
There will be no (major) hermetic breakthroughs, other than those that are important to the story. That doesn't mean you can't show initiative in that direction - the story in the end is not mine alone, but a joint endeavour. But there will not be the story about the Bonisagus who sat in his lab working towards the great discovery X. On the flip side, you might get to play a Bonisagus that happens to break one of the major limits of hermetic magic - if that is part of the grand finale of the story.

So, if telling a story is your thing and bookkeeping is not, would you be interested to see this come to life?

I have no clue yet what the story would be about or where it would take place. It's probably going to be more about Magi and less about companion or grog player characters. And as I said initially, if there is interest and I pick it up, it's going to be quite a while until you would see an actual ad.