@Tamasock: I believe it's really just for bragging rights, and showing off in your Music City Apartment. o_O
Yes, I do believe you need around 50,000 CD EXP to get your CD to 4th Gold. Not 100% sure about the others, but I think that you need 100,000 CD EXP for a 3rd Super Gold CD, 200,000 CD EXP for a 2nd Ultimate Gold CD and 300,000 CD EXP for 1st Gotchi Gold. If you guys are wondering, the '1st Gotchi Gold' icon is a Golden CD with a Gotchi Crown atop, and maybe some sparkles.
I might make a list of the amount of CD EXP you need to upgrade your CD Rank later. But, as I'm already a Level 50, I wouldn't know how much Producer EXP you need to Level Up, though. =[