It is great that somebody is trying it.
Just because of symmetry you would think that if one ball is different to the other 3 (either heavier or lighter) it would only spin balanced on that one. This does not necessarily mean that the pyramid will flip 180 degrees to that position when started with the odd ball up. So I am curious to know what you get.
With wood or plastic balls, it would be possible to drill a hole in one ball to make it lighter, or fill it with some denser material to make it heavier, keeping all the balls the same size.