material.renderQueue = 4000 has no effect.

I need an object to be rendered on top of all others in the scene. I read that I could do this by setting the renderQueue property of my material.

GetComponent<Renderer>().material.renderQueue = 4001;

Has no Effect at all. Other objects are still covering my object. I set this within the Awake() function of a script attached to the object I want to be on top.
Is there something else I have to do?
(and no other object have not the same material)

The Rendering Queue doesn’t quite do what you think it does. While it does change the order in which objects are rendered, that doesn’t change the process of determining whether to render them in the first place.

What you’re looking for is part of shader code instead. In this case, this is a matter of changing the shader from the default of ZTest LEqual to ZTest Always. This will result in the object always rendering, no matter what’s in front of or behind it, rather than only rendering if it is in front of another object.

That said, that doesn’t mean that the Rendering Queue isn’t a factor in this. If you have two objects, each set with ZTest Always, you can then be certain one will always render on top of the other by changing their position in the Rendering Queue.