You can't directly compare a 5004 and a 5951! The 5951 doesn't replace the 5004 and is much more refined technically speaking!
The calibre is much thinner!
Of course the design is another aspect and I will not discuss this aspect. Just that the 5951 is much more elegant on the thcikness side!
Cheers
Dje
ps: regarding the prices you mention, I've had inferior prices indicated, around 400 000 and 500 000 Swiss Francs if I remember well, but please not take them for granted!
... after having looked at the details I understood why 5950 and 5951 are that expensive: Same Caliber as 5959, so they cannot be compared with the thick 5004. Only very few pieces made.
Maybe we will see a CH 29-535 based split seconds chrono one day which should be affordable I hope...