Just to compile all of these with maps:
I will try to see if there are any others where there are 4 WHS within marathon distance (42.195 km / 26 miles 385 yards).
Brussels (1 & 1.5):
4 WHS, 1 t-list, 13.4km or
top it up to 4 WHS, 2 t-list, 14.7km (Primeval Beech Forests (Sonian Forest), Stoclet Palace, Horta Town Houses (Van Eetvelde), Grand Place +
Galleries St Hubert & Palais de Justice)
Brussels (2 & 2.5):
6 WHS, 1 t-list, 36.5km or top it up to
6 WHS, 2 t-list, 37.8 km (Grand Place, Horta Town Houses (Van Eetvelde), Stoclet Palace, Primeval Beech Forests (Sonian Forest), Flemish Béguinages (Groot Begijnhof, Leuven), Belfries (St Peters, Leuven) +
Galleries St Hubert & Palais de Justice)
Mexico City:
4 WHS, 2 t-list, 19.2km (UNAM - Casa Barragan, Historic Centre of Mexico City, Camino Real +
Museo Casa Estudio Diego Rivera and Frida Kahlo & Chapultepec Woods, Hill and Castle)
Paris (1 & 1.5):
4 WHS 20.2km or
4 WHS and 2 t-list 23.6km (Versailles, Le Corbusier (Immeuble locatif à la Porte Molitor), Paris, Pilgrimage Route to Santiago in France (Tour St Jacques) +
Hangar Y and Office National d'Etudes et de Recherches Aérospatiales, Meudon)
Beijing:
4 WHS, 1 t-list, 20.9km (Summer Palace, Grand Canal, Forbidden City, Temple of Heaven +
Central Axis of Beijing)
London:
4 WHS, 24.9km (Kew Gardens, Westminster, Tower of London, Greenwich)
Seoul:
4 WHS, 1 t-list, 29.2 km (Namhansanseong, Royal Joseon Tombs, Jongmyo Shrine, Changdeokgung Palace +
Seoul City Walls)
Rome:
4 WHS, 1 t-list, 36.5 km (Villa d'Este, Villa Adriana, Rome, Vatican City +
via Francigena)
Jerusalem:
4 WHS, 1 t-list 40km (Hebron, Battir, Bethlehem, Jerusalem +
Jerusalem (Israeli tentative site))
And just in case there is any movement on these tentative sites I've mapped some potential future option that will smash the current Brussels record:
Esfahan:
3 WHS, 2 t-list 6.9km (Persian Garden (Chehel Sotoon Palace), Meidan Emam, Masjed-e Jâme,
Vank Cathedral & Industrial Heritage of textile in Iran (Risbaf Cotton Factory)) 5.4 km for 4 WHS if the industrial heritage of textiles gets inscribed.
Valletta:
3 WHS, 1 t-list 5.2km!! (Megalithic Temples (Tarxien), Hypogeum, Valletta +
Knights Fortifications (Floriana))