Basically, the smaller the group the more 'hybrid' the classes (to double up on the missing chars). The larger the group, the more 'pure' the classes. IMHO a 4 man is just on the crux between between being called a small or large group.
I think the pure classes will do fine with this method, there is no reason why I can't fill the rest of the group with other real players, trying to find a balance of what is flexible and what isn't when adding just random players into the mix. Tomorrow I'm going to try to test out melee while on auto follow and see how that works, will make an easy decision for me if multiple melee is a reality or not