There are many ways you can divide the costs, however the most proportional would be a Shapley Distribution. In your case, if you only have need for 400 feet, that works out to 1/10 of the total cost as stated above. The other shares would have to be worked out according to the feet of road necessary for each property to use.
Having said that, it could be argued that you could access the road anywhere on your property, so the 400 feet you say is all you use could be a point of argument for the others. They might say your share should include the distance to the end of your property.
I live on a road where all owners share equally, and I don't argue because I'm at the end of the road and would wind up paying more if it was actually divided proportionally as described. If we were to divide it as described (Shapley Distribution) I would probably argue that the road proportions should be measured to the farthest point on the persons property where they "could" access the road.