It depends what you're trying to achieve. Silent coat will very effectively stop panel resonance - and it's actually the aluminium side that does most of the work (converting the vibration into heat), the mass comes from the bitumen layer. So 4mm will add more mass for the same area, or the same mass for smaller obviously. I couldn't tell you what the best coverage is because I've not done it.
If you're looking to reduce road and airflow noise you'll need some sound absorbing material on top, deadening reduces it but is not the most effective for road noise, i.e. people cover panels in it simply adding mass and it would be more effective to simply add foam, less weight and probably cheaper.