I think each of these embodies a sense of personal liberty. The society of Fahrenheit 451 is not what I would call a "free" society. The citizens are watched and are constantly bombarded with propaganda. Mildred is constantly pressuring Montag to get another "screen" even though the previous wall screen set them back economically. She doesn't seem to care or notice.
Clarisse, the old woman and Faber offer a more "real" sense of humanity when compared to Mildred. They are able to carry on conversations that make sense. They are not listening to shell radios while having a conversation. And they seem to have a more personal sense of who they are in the face of the current social trends.
Once Montag meets Granger, he has a profound sense of knowing himself. He has "arrived" at a place intellectually where he can see what is wrong with his society and decides to become a book himself. These persons hold out hope that knowledge and what is found in books will still be around after this government is destroyed. For Montag, these 4 hold out personal freedom and liberty. They hold out for individuality and non-conformity regardless of the personal cost.