If we're to capture an image of a body using four (4) WebCam with a 16 megapixel (1 in the front, 1 in the back and 2 sides(left and right)). Is it possible to make a 3D out of it using image processing?
You can make a 3D model of the scene (<=> the depth) if the two cameras are looking to the same scene. After that you detect/extract the commun points between the two images and do some triangulation so you can recover the z component. From your installation as you discribe it you can only enlarge the field of view
If you want to do this seriously, you should have a look into photogrammetry techniques - or rethink you problem and find out if you really need a full 3D model. For photogrammetry, I assume you would need more than four images.
To convert from pixels to centimeters, a simple scale in the scene you image would be necessary. I think this is the simplest solution to the conversion problem, and since you are obviously free to design the scene where and how the images are acquired, it certainly is the simplest solution.