The ColumnStore 120™ is essentially four ColumnStore 30™s plus three brackets, so they are no different except in the way they are packaged for shipping.
A four unit high stack will not be quite as stable as a two or three unit assembly, but it won’t fall over because the brackets are designed so it can’t (unless something leans against it). Overall stability of the unit will depend on the surface it is placed on as well. Normally for ColumnStore 120, this would be floor standing but it needs to be a smooth surface.
It is important to note that the feet supplied with the units are only placed on the bottom unit and not the other units in the stack which sit flush with each other in the mounting bracket and that will contribute to the stability as well.