hamilton33's subsequent response via email:

We're not looking to "protect" pages, but just limit certain editors from viewing the posts in the dashboard.

i.e. Category 1, Category 2, Category 3 (each with 5 posts)

We want User1 to log in with his username/password, but he can only see/edit the posts in Category1. Same deal for User2 and User3.

We've been able to accomplish this… but the problem is when this is all in place, trying to browse to pages when not logged into the dashboard results in 404 errors.

I get what's happening… it's not just protecting visibility in the dashboard, but also visibility on the site. But we only want the former.

You should get the desired functionality (restricting wp-admin access only) by removing the "Post Reader" and "Private Post Reader" category restrictions. Just leave Post Contributor, Post Author, Post Editor and Category Manager restricted.