As Willys has stated on average a Tunnel Ram is designed to a more specific power band than a single or dual plenum single 4bl intake manifold.
There are pros and cons to both set ups. It's all a matter of what's going to work best in the specific combination. One of the problems you can run into with a Tunnel Ram is getting it sized properly for the specific combination. Unless you're going to build the runners and plenum yourself they probably not going to be optimal for the combination you're running, and then you have to crutch with the carburetors. I prefer to not use vacuum secondary carburetors in a 2x4 application due to fuel distribution issues. If your front two barrels are wide open, and the rear two only open as much as is needed you will get different amounts of air and fuel to each cylinder which is less than optimal.