Wow, Retroman, you are really one lucky guy, man !!!
Please, please take good care of these disks, and when you got time please transfer them to the PC.
4000 disks indeed is a LOT ! I have collected C64 games for over 15 years and my collection was about 2000 disks in the end, having more than 5000 games. Also I know some of the biggest collectors around the globe and I don't think anyone had more disks !
Very rare games I would say are Murder by US-Gold, Daffy Duck by High-Tec and Megatwins.