RGB LED controllers probably won't work for what you want to do and custom LED controllers seem expensive. If your project can utilize 120 volts AC then I have a suggestion. There are inexpensive four channel dimmer packs for 120 volt incandescent lighting. If each of your rope lights has or can accept a 12 volt power supply transformer then you can plug the rope light power supplies into the dimmer pack. The 136-DMXQ-08 from Cheaplights.com will work well. Check it out:
http://www.cheaplights.com/eshop/product.php?productid=11379&cat=0&page=1They have other controllers as well. Good luck with your project.