Should I tell my mortgage brokers anything?

I am in the process of purchasing a home, and I have obtained pre-approvals from four different brokers. Once I am under contract, then I will lock in the rate with the one broker who will provide me with the best rate on that date. Right now, none of the brokers is aware that I am working with other brokers. All of them have been extremely helpful in the process so far. As a professional courtesy, should I let each of them know in advance that I have other brokers lined up and may not end up choosing his/her firm once I am under contract? I want to be professional with all of them and so the last thing I want is to mislead them in any way into thinking that it's a done deal and they are the chosen one. At the same time, I would think that they should be aware of this in their line of business that borrowers shop around all the time. Thanks!