Although heavier than the MB20, I have found that using the MB21 provides a much better shooting platform, i.e better stability. Not always and not in all situations, but when weight is not an issues (like not climbing, not walking miles, not running, etc) the F4 with MB21 is really a pleasure to use.
That said, I will never mount the F4 + MB21 on a tripod, again; did it, was lucky to catch the tripod and camera before it hit the ground. It was probably my mistake, but even so, high tripod and so weighty a camera requires attention and experience (I have the first, lack the latter).