I'm confused, how does adding 4 to 4 get you to 16? And in this case you'd be cheaper to buy a 16 license, blow your install away completely and start a new one.
Glad you asked this question though, this is exactly what I was debating. I currently have 4 cameras and am getting ready to purchase a license and was debating whether to go with the 4 or 16 for future use. If it's going to cost me more money to "upgrade" later than it would to just do it now, I guess there's my answer.