Phil,

"One of my concerns with multiple portals is managing user accounts across the 4 portals.  How does someone move from one portal to another after they have logged in?"  Depends if you are using AD authentication or not - if so it came be seemless once the user is setup correctly. If they are logging in with standard DNN authentication I think the only answer is another log in but I am not positive on that.

Like Mike said there are issues with going with multiple portals and there are some work arounds that can fix most issues. In my current situation I have mulitple child portals (parent is disabled due to cookie issues from jumping from parent to child) for distinct geographical regions all using AD for authentication. The only issue at the moment is users who need access to more then one portal - have to be manually be given access to the proceeding portals.  If I had it to do over again I would probably have gone with one portal. Here are Pros and Cons in my experience (Mike feel free to add to it)

1 Portal (reverse them for multi-portal)

PRO
 - Not as many authentication issues
 - 3rd Party Module licensing simplified (cheaper)
 - Less issues at the technical level of administration

CON
 - Sure volume of users, security roles, and pages under control of one admin account.
 - Not as simple/clean with mixing Authentication types
 - More training needed for users to understand security roles
 - Blanket security roles across portal doesnt work
 - There has to be a trust level across users with elevated/admin priveleges (admins of one divisions dont mess with another divisions content)

Hope this helps!

Craig