You have fewer rights at a border crossing for sure, but that doesn't make it some sort of "constitution free zone", mostly it means that they are allowed to conduct 'routine' searches and interviews without a warrant, and hold you for a certain amount of time, and deny you entry if they deem that necessary, all without a judge or warrant involved. How much protection you get from the 4th and 5th amendments is definitely different at the border than not at the border. But that does not mean you have zero rights at the border, they just want you to feel that way so you stay compliant.