Ive owned a 1DS3 for 5 years and its one amazing bit of kit. Its been in rain, snow, deserts, has been dropped onto rock and tarmak 5 times and still works perfectly. It does what it does and was designed for very well and keeps going where only pro bodies go.Colour renidtion is excellent, autofocus is good. Its used for landscapes and the odd portrait where its not going beyond iso 400. It can go higher but i dont need to. So buy one, its built like a tank, will last a lifetime in all conditions but its slow shooting and limited iso is its achillies heel. And if you loose your hammer you can knock nails in a wall with it.
As for why the 1d4 has kept its value is easy to explain. Its the crop factor. Not wanting to buy bigger lens's as they cost so much to negate the loss of crop is to much for many so the prices are holding up well. People want that crop factor.