Two images found on China's version of twitter called Weibo provides a hint on what the Transformers 4 version of Bumblebee and Optimus Prime may look like in robot mode. While the labels on the image indicate Transformers: Prime, if you look at face mask and costume details, they are clearly of the movie aesthetic. Since these are costumes, you can't really leap to a conclusion on what the final result will but it does indicate that their designs have been mostly tweaked rather than completely changed.