The reason why your equation is failing is that you are using the syntax that grass uses.
dem[4,0] means nothing in ArcMap and is an invalid syntax.
Reading the web page you linked to it seems that this syntax is all to do with calculating some off-set.
I'm not sure if this is a red herring as my raster processing skills is not as good as my vector skills but I know you can shift a grid using the Shift tool. Or maybe someone can suggest some whizzy way of using focal stats?
Duncan