When you say vibration, is it movement of the base parallel with a particular axis or resonance of structural elements when the machine runs or bending/twisting of elements?
To work as a brace, 4" is very little height and should not help much. It shouldn't matter what the section is (strip, round or square) as that member would have only axial loads in it as a brace. For best result, you should bolt it diagonally from top of one leg to the bottom of the other to triangulate the base.
It might help a bit but you wouldn't be fixing the problem which seems to be a too flexible base.
What is wrong with spoiling a spoil board? It is a $30 piece of MDF meant to be spoiled and replaced as it wears down. Besides, wouldn't you have to cut into the spoil board anyway to separate a piece from stock?