As with all activations, the size of what you use should be proportionate to the space. Four inches sounds as if it might be a little small, but if it seems adequate to you, go with it. That's the ultimate guideline, how does it resonate for you and does it represent what you want to manifest in your life?
As it is already gold, that may already feel as if it represents the abundance you seek sailing into your space. If not, maybe you could find some smaller gold coins at a toy or party store, or mini crystals or gem stones.
Don't forget, too, that you can use muscle testing to see what the energetic impact of an activation is. If it's not as strong as you would like, make it bigger and bolder.
All the best!