Which saddle for London to Paris

I would guess not many can ride for 4 hours without some sort of discomfort in the seating area

Depends, innit. Three years of Dunwich:

1) Ludicrous folding bike (Dahon Helios SL). Arse in agony, nerves in hands shot such that I can't feel a thing in my little and ring fingers for weeks.
2) Orbit Photon steel audax frame, carbon forks, Koobi Xenon saddle. No actual agony, but definite tenderness. Hands fine (but frozen because it rained all the bloody way).
3) Custom 631, carbon forks and seatpost, Koobi Xenon saddle. Really, genuinely not a twinge. Tired the next day, but blissfully free of pain.

Same (£20 Altura) shorts every year.