Combining multiple layers in a new layer?

how do I “combine” (clearly not the right “verb”) four existing layers onto a new layer while retaining the original four layers? thanks

If I understand correctly, you are trying to duplicate the combined contents of 4 existing layers, into a 5th new layer, while maintaining the 4 existing layers intact. So you will now have duplicate objects in your file. Did I get that right?

In that case, unhide and unlock all the objects you want to copy over from your original 4 layers. Then, select the 4 layers, and right-click:select objects

Then all your objects form your 4 layers are selected. Go to your destination 5th layer and right-click: copy objects to layer: