Your cardiologist should have a clear understanding of all your risk factors and, after indexing, has determined that 3.9 cm is safe enough in your case to allow you to exercise without lifestyle recommendations.

I'm at the point where I'd be willing to sell my soul to the devil just to get this same advice.

I would recommend learning to pay close attention to your breathing during exercise and to avoid Valsalva maneuvers.
This is to avoid further growth in the future as much as possible