MichaelBay.com has confirmed that the yellow and black vehicle photographed at Monument Valley Navajo Tribal Park where Transformers 4 is currently filming is the alt mode of Bumblebee. The site says 'Bee is a "highly modified, vintage 1967 Camaro SS." Hi-res images of the car can be found here.
What kind of customizations have been added? Pro-Touring forums might have some ideas as they have noticed that the custom design of Bumblebee seems to match what "Mr Vengeance" built two years ago for his '67 Camaro. The details on his mods are here. Thanks to J1000, Tony and others for the links.