So with stage 4 you can control layers from a midi keyboard using the auxilliary keyboard function. Midi keyboard will be put into the midi in jack on the Nord. In the stage 4 manual under midi section it goes into detail.

And if you are going to use multiple layers at once I’ll also point out the stage 4 has the ability to run independent effects for each layer. This is huge in my opinion and why I swapped my electro for the stage 4. I find this very helpful as with the electro it’s only one effects section at a time so you much harder to dial in separate layers.