As Dan said you can do it but 4 OPS for a 9A attack (assuming no trench)??
Of course there are exceptional cases when it might be vital to do so but it is a potentially a big waste of resources.
Sorry to raise this when your question was only about the rules but the longer I have played POG the fewer 3 and 4OPS attacks I have seen. Multinational attack is one of the reasons why you might have such an expensive Activation but that is a reason to avoid multinational stacks as much as possible.
Right, its not the best use of Ops but in some cases (like meeting a MO) it can come in handy.
Just fyi...trenches don’t impact multi-national attacks. Entrenched defenders nullify flank attacks...which may be what you are thinking.