Trying to keep this absent of political boundaries, so this comes with no view at all on where such boundaries lie in reality, in law or in an idealised world, but I think Hebron to Jerusalem is in theory possible, though in reality almost certainly impossible.

This Route is just over the qualifying 42.159km. It is meets the criteria of being inside the core zone (but only just at their extremities) though I think falls just short of the Eastern boundary of Battir in Bayt Jala, but the core zone map isn't 100% clear, and trying to get a few hundred metres up the road suddenly forces an 8km detour.

However, it forces a big loop to get out of Hebron which adds about 4 km to the total, without that it would comfortably be under the qualifying distance. It looks like the 3557 out of Hebron crosses through several checkpoints and guard posts, so I assume there are practical reasons for the diversion.

However, I think it is technically possible to cover the 4 core zones in under 42.159km by shaving some parts off even if that loop out of Hebron must be kept.

I really like your path meltwaterfalls. It is literally fantastic :)
When I visited Hebron in 2015, the only in and out to the city was through 3268. All other ways that we had tried were somehow blocked. The road 3555 is also closed if you look closer.

https://maps.app.goo.gl/gWZQJxnMMGB9xXC57

Yeah, 100% a fantasy and requiring some sort of diplomatic passport

Thanks for flagging up the barriers on the 3555, I guess this one probably will never be run in under 42km but the workings are there should the situation ever change I guess :)

Found another one, just squeaking in

Utrecht- Amsterdam: 4 WHS, 41.6 km (Rietveld Schröderhuis, Lower German Limes (Utrecht Domplein), Dutch Water Defence Lines, Amsterdam Canal Ring)
Unlike many of the others this has the potential to be quite a nice run, that majority of it being on the footpath next to a canal/dijk. Though it does just run through the Utrecht Domplein, when the OUV is actually all underneath the surface, but hey ho why let that get in the way of a nice run, and the Cathedral is much nicer than a series of small walls underneath it.
You can shave off a km by not following the canal up past Nigtevecht, and heading in via the Johan Cruijff Areana if that takes your fancy.

I checked and the Wadden See core zone is too far to get in 4 WHS (+Amsterdam, Beemster Polder, Dutch Water Defence Lines) if running to Amsterdam from the north rather than the south.

LOL

Still need to turn this into a connection I notice.

meltwaterfalls
Found you another one! I was trying to see if I could fashion one for Ghent for August with de Vooruit falling nicely on a 1.6km route from the Belfry to the Petit Beguinage (I couldn't). But Antwerp provides a 20km route between the Stadhuis (Belfry), Plantin-Moretus Museum, Maison Guiette and then out to the Beguinage in Lier. In fact, choosing Lier as your Belfry option instead of the Stadhuis might even allow you to shave off a few more metres..

(In the other direction Plantin-Moretus - Stadhuis Belfry - Hoogstraten Beguinage - Wortel Colony of Benevolence is closing in on 40km)

Sorry I completely missed this one, thank you.
Antwerp-Lier: 4 WHS, 1 Tentative 19.7km (Belfries, Plantain Moretus Museum, le Corbuseier, Begijnhofs)

I will have to rationalise these on a map somewhere

Yes!!
For those that came to the Brussels meetup this year and saw me hobbling about, they will have know that I ran one of them Brussels (1): 4 WHS, 1 t-list, 13.4km (Primeval Beech Forests (Sonian Forest), Stoclet Palace, Horta Town Houses (Van Eetvelde), Grand Place + Galleries St Hubert)
I say ran, I actually walked a considerable amount as I picked up an injury, and I'm just not as fit as I was 14 years ago :)

But the time to beat is 01:27:47 (rather slow so easy for anyone reasonably fit)

But now much easier if you are in Beijing ....

By far the shortest route is now:

Beijing: 4 WHS 8.6km (Grand Canal, Forbidden City, Temple of Heaven, Beijing Central Axis), In fact this one can be run without leaving the Central Axis core zone.
Depending on pedestrian restrictions/ ticket queues it could actually be a reasonably nice run, unlike the slog out to the summer palace. It may well be short enough that the pollution wouldn't leave you with black lungs afterwards.